Random Encounters: Mechanics and the Optimal Strategy!

I've spent most of the last week running experiments and gathering data to figure out how random encounters actually work in this game. Considering that random encounters are probably the greatest threat to difficult runs like Ironman Cataclysm, I was surprised how much of the system was still a black box, with even the wiki being badly wrong about it.

Perhaps the biggest misconception: Stealth is completely irrelevant.

I've now figured most of the random encounter system now, including how ambushes are determined, and the parts needed to make strategic choices to optimize builds to survive random encounters, and what travel pace to choose. After testing hundreds of random encounters across different parties, here's the TL;DR:

• Surprise is Perception vs Perception
• You need to win that check by 5 or more to trigger surprise
• 25% of encounters are potential attacks in your sleep
• Fast travel is actually safer than Slow
• Oblivion Clerics reduce your chance of being wiped out by random encounters by about 1/6 overall

Now here are the details!

Not Truly Random:

There are several ways in which random encounters are. The most important (and one I think many people know since the game strongly implies it in the tutorial) is that you can never get two in one day. Which means you're free to fire away with 100% of your daily resources in every random encounter, no need to save any in case of another.

One thing that does seem to be random in my tests (though impossible to be completely certain of) is that any of the random encounter enemy groups can be fought either on the road or in camp at any hour. Darkvisionless human enemies are frequently battled in the dark, necromancers and their undead hordes or deep spiders that are disadvantaged in daylight are frequently fought in the day, etc. Sometimes I've faced the exact same enemy configuration (such as Goblin Shaman +3 Dire Wolves) multiple times in a row with one time being during travel, another in camp, another trying to attack in my sleep, etc.

How common are encounters? Well they seem to be substantially more common in some regions than others. The journey from Caer Cyflen to the Dark Castle is similar length to that from Caer Lem to the Lava Forest (about 12 days each at a slow pace). But I averaged about 1 encounter per 3 days from Caer Cyflen to the Dark Castle and back over many trips, vs less than 1 per 6 days from Caer Lem to the Lava Forest. The difference is not only statistically significant but strategically significant! It makes a certain amount of sense, of course, that a mostly desert area like the route to the Lava Forest would have a lot of empty space, but any instances of making sense are strictly coincidental in this system!

Even given the greater or lower encounter densities in certain regions, I don't ever seem to observe the kind of hot streaks of lots of encounters one day after another that one would expect sometimes if it was truly random. So I suspect the minimum amount of time between encounters that the game is enforcing is higher than just resetting the clock the instant you wake up, and indeed it might be higher than 24 hours. Speaking of which...

Time is an Illusion, Naptime is a Particularly Damaging Evocation.

Speaking of hours, when in the day encounters occur isn't really random either, and this is strategically important. You spend much less than half the day actually traveling, and thus more than half the day in camp. But nonetheless, fully half of all encounters occur while traveling and the other half in camp. Statistical significance with more than 99% confidence that it's half and half. So at some point the game seems to be deciding there'll be an encounter at some point in the day and then doing a coin flip to decide if that's going to happen during travel or camp.

On the other hand, the % of encounters which occur while the party is 'resting' and is thus susceptible to the infamous "One party member surprised, everyone else asleep" encounter scenario does seem to be proportional to the amount of time spent sleeping. Regardless of amount of time in camp, the party seems to spend about 6 hours asleep. That of course is 25% of the overall day. And my data has almost exactly 25% of all encounters occurring during that resting phase. That may just be a coincidence though, it's possible the game just flips another coin after deciding on a camp encounter to decide if it's an encounter while you're resting. Whichever is the reason in the logic, the result is the same: about 25% of all random encounters are potential attacks in your sleep.

Tangentially- the number of hours spent 'resting' on the travel screen does NOT actually seem to vary with the number of elves in the party. I've timed it a few times with parties of pure elves, parties of pure non-elves, and mixed parties. They all always take about 6 hours. In terms of being productive though, elves do get some extra time crafting thanks to only needing a 4 hour trance instead of a 6 hour sleep. Have an elf and a non-elf both start crafting identical big things and count up the craft checks in the log after a rest, and you'll see the elf is making an extra 2 checks per night.

Tangent over, what determines if you get one of those brutal random encounter attacks in your sleep where three people are asleep and the other one is surprised (presumably surprised that his companions are sleeping so soundly that a battle and monsters roaring 5 feet away doesn't wake them)?

Well, it checks if you're surprised (more on that in the next section). If you're not surprised, then you get an ordinary battle in camp and no one is asleep. If you ARE surprised, then the outcome depends on how many elves you've got. If you have exclusively elves, no one can be asleep, so you just face a normal surprise attack. if you don't have any elves, then you get the 3 asleep and 1 awake situation.

If you have a mixed party of elves and non-elves, then things get a bit odd. If the game was truly being random about picking someone who is on-watch to be surprised while everyone else is asleep, then we should expect situations where one of the non-elves is awake and surprised, the elves are awake and surprised, and the other non-elves are asleep. But... I never get that outcome. Non-elves being asleep is all or nothing. The sample size here is too small to have a very high confidence level, but it looks to me like if you have a mixed party, it isn't actually checking if one particular non-elf is awake. It's just deciding randomly to have all non-elves asleep or all non-elves awake but surprised.

Surprise, Surprise

THE most important factor in whether you get slaughtered by an early random encounter and lose your run is not your tactics. And it's not even just having all elves to negate sleep attacks in particular because, although those are very deadly, they're overall quite rare. It's whether you get surprised or not! There are quite a few encounters you can get at level 3 that are statistically likely to down 3 or 4 party members before you can even make a move if you get surprised, and they can do that to you just as well whether you're traveling or in camp or resting.

Unlike in normal battles where it's possible for just some creatures to be surprised and others not surprised, it seems to be all or nothing in random encounters. An entire team is surprised, or an entire team is not surprised.

So what determines who gets surprised?

Perception.

And nothing else.

Seeing is Achieving

You might logically expect (and the wiki says, and 5E's rules say) that Stealth would have something to do with it, but it doesn't. Stealth is completely irrelevant.

Some tests with the cheat engine demonstrated that a party where everyone has -30 Perception and +30 Stealth never surprises any encountered enemy, whether traveling or in camp. Indeed, that party is surprised by every enemy, whether traveling or in camp. Conversely, a party with +30 Perception and -30 Stealth always surprises every enemy, whether traveling or in camp. You don't need to be sneaky to sneak up on someone! Seeing is deceiving! And therefore achieving too. A surprised enemy group is one you can either kill or skip fighting entirely if they're too powerful to risk.

This is just another thing that makes Sylvan Elves an incredibly good race, since they not only get Perception proficiency and immunity to sleep but also bonus Wisdom to boost that Perception up even further. (And also get a pile of other useful perks of course).

Now, for almost every other skill roll in the entire game, Solasta will pick whichever party member is the best at that skill and have them roll it. Convenient!

But these perception rolls to surprise the enemy or avoid surprise are different. It picks a party member at random and has them do perception. Which means that whereas for most skills it is optimal to have exactly one party member who is good at that skill, for perception you want EVERYONE to be good at it. Having even a single party member who is bad at Perception is a weakness that gives you a chance to be ambushed and slaughtered, no matter how good everyone else is.

Travel Pace sometimes affects things here. It has zero effect on the half of all encounters that happen in camp. But if the coin flip decides that your encounter is going to happen during travel, you roll Perception with advantage if going at a slow pace and with disadvantage if going at a fast pace.

Eagle-Scout Scouts?

Now what are we rolling perception against? Well it looks like the enemy team is doing the same thing we are. One of them seems to get picked at random to roll their perception. Stealth isn't relevant for them either!

This is quite easily demonstrated because there are a ton of common enemies such as (most) goblins and spiders that have very high Stealth but bad Perception. If the enemy used their Stealth, those guys would be good at ambushing, but they're not. I wrote down the Stealth and Perception of every monster I encountered (Except Dark Apprentices. For some reason they don't show up in the bestiary and I can't find their Perception online), and my results show that high Stealth low Perception foes like goblins and spiders are some of the easiest to ambush.

It was trickier to find evidence about whether the enemy is picking a random party member like we are, or whether they get to pick their best. There are only a tiny number of encounter configurations (Such as Badlands Eagle + orcs, or Scout + Darkweaver + humans) where one creature on the team has vastly superior perception to the others. So my sample size for this isn't as big as I'd like, but the results of those encounters indicate that I surprise them as often as I should if the game is picking randomly, rather than struggling to surprise them because Badlands Eagles and Scouts have good Perception.

The Boring Middle

By now, you're probably thinking something like "Wait a minute, you mean if our perception beats their perception, then we surprise them? That can't be- what about all the times when no one is surprised!"

Correct! After waiting the demanded 10 rounds, here's your answer: we surprise the enemy if our perception beats their perception by at least 5. We get surprised if their perception beats ours by at least 5. And in the middle, to no one's surprise, no one is surprised.

I had to get quite a big sample of encounters to test this, but the data matches this model with remarkable closeness. And it's also not surprising that the designers picked 5 for the required margin of success, since 5 is a fairly important number in 5E systems.

So in summary:

The game decides there's going to be an encounter today, and basically flips a coin for whether it happens during travel or in camp. If in camp, it also determines if it happens while you're resting and thus is possibly a sleep attack. Regardless of when the encounter will occur, the game picks a random member of your party and a random monster to roll perception against each other. If this encounter is during travel and you're at a slow pace, you roll with advantage, and you get disadvantage if going at a fast pace.

If your random guy's Perception beats their random guy's Perception by 5 or more, your team surprises their whole team and can either 'hide' from them to safely skip the encounter, or battle their whole team with surprise. If they beat you by 5 or more, your whole team is surprised. And if it's during the hours of rest, your non-elves may be asleep as a result. If the difference between Perceptions is smaller than that, no one is surprised and both sides just fight it out.

An I for an Eye

Obviously, different monsters have different Perception scores. But how different? What's a normal enemy perception score we have to try to beat?

Well it's 1. Roughly. Maybe 1.5 depending on the area. And it doesn't really even scale up as you level up. If anything, it might actually scale down a little bit because the most perceptive enemies are things like Scout (+5) or Spy (+6) that are really weak and not common outside the beginning areas. Almost every other enemy is just in the 0-2 range. Some things like Ogres go as low as -2 and can be found in even pretty late areas.

So if you could reliably get a Perception result of 17, you'd expect to ambush over half of the enemies even without advantage from traveling slowly.

The trouble is... you can't reliably do that. There just aren't many ways to boost your Perception in this game. Let alone to do so for the entire party. You can (and should) have Proficiency in Perception on absolutely every party member, but that still just gets you 2-3 for most of the game. And there aren't many ways to boost your Wis either.

I can't find any items that boost Perception or Wisdom except for the attunement-requiring Cloak of the Ancient King which you'd have to craft yourself after getting to Brotherhood with the Tower of Knowledge, giving the wearer 19 Wis. Not the kind of thing you can/want to outfit your whole party with. I guess if you had infinite money (from crafting Tiger Fang, also available at Brotherhood from the Tower of Knowledge), you could theoretically get 4 of those and short rest to attune one to every character before traveling. But... if you have the resources to do that, you're at the point in the game where you've become so powerful that random encounters stopped mattering anyway.

But even if you DID have 19 Wis and a lategame +4 proficiency in Perception on every character, I want to make the point that that still leaves a LOT of room for even an enemy with a lousy 1 Perception to surprise you. 1 d20 minus another d20 has a lot of variance, and it's not exactly a rare event for you to roll 1-4 and the enemy to roll 16 or so. So... you can tilt the odds substantially in your favor but you really can't make yourself safe by just boosting Perception

Fantasy Settings:

Since Perception is an ability check, you might think that the setting that gives allies or enemies a boost to ability checks would affect the chance of surprising each other.

Nope.

That setting is completely ignored. Which, incidentally, seems to mean that there's no difference in the likelihood of being surprised on Cataclysm (where enemies normally get +3 to ability checks) vs other difficulty modes.

Brave, Brave Sir Goblin

If you win the surprise check, you have the option to just hide and automatically skip the battle. The enemy doesn't play by such rules though, and will automatically skip battles without even doing a perception check, let along beating you in one.

You've probably noticed that you sometimes see a travel log entry saying something like "Weak monsters see the party coming and run away". Don't be fooled by the wording about seeing you. Perception is irrelevant, that's just flavor text.

It's likewise pretty much just misleading flavor text that the monsters are weak. Monsters 'running away' has little to do with whether that monster has any chance against you. It's all about your level, and what area of the map you're in.

By around level 6, all encounters in the vicinity of Caer Cyflen will stop, even though there are some monsters you can encounter there like Warlords or Necromancers that are still quite a serious threat... and the exact same warlords and necromancers will still fearlessly attack you in other parts of the map. Likewise my level 10 party gets attacked by pitiful groups of goblins around the Lava Forest. That region is considered a high level one, so random encounters won't be skipped even if they have absolutely no chance against you. While Caer Cyflen is a low level area, so random encounters get skipped even if they're actually still a threat.

In any case, if you see a skipped encounter in your travel log, the good news is that that still counts as your random encounter for the day! You can't get hit with a real encounter right after your party was apparently oblivious to another.

Church Elder Scrolls IV: Oblivion

What about Oblivion clerics and their Peaceful Rest feature?

Well, reflecting the dual aspects of their goddess Maraike, half of the description is total lies, while the other half is true!

"While camping, reduces the chances of being attacked and prevents from being surprised."

The first part is completely false. In all my tests, the frequency of camp encounters remains the same (50%) as does the frequency of encounters overall in any given area.

The second part is true though! Contrary to something I've heard some people say, about it only preventing you from being caught asleep, but still allowing you to be surprised, this feature truly does completely negate any chance of being surprised in camp. You can give your party -1000 Perception with the cheat engine and you'll get constantly surprised while traveling, but if you have an Oblivion Cleric in that party, they will never get surprised in camp. Instead all those encounters that would be surprises against you just become neutral encounters with no one surprised.

How useful is this? Well... quite useful, but it depends on your party! The worse your party is, the better it gets. If your party has bad Perception or people who are not elves (...that Venn diagram is a circle), then a ton of surprises are going to be negated and turned into neutral encounters instead. But if your party are all unsleeping elves with high Perception, the benefit is smaller.

This one feature really does justify strongly considering them on an Ironman run, despite them otherwise being a mediocre kind of cleric.

Just remember that it only helps with the half of encounters that are in camp, not the half while out traveling. You're still just as likely to be surprised in those.

Maybe the Real Treasure was on those Adventurers Waylaid On Their Way

So there are basically 4 things you can do to reduce the odds of random encounters wiping out your ironman run:

1. When building your party, make sure they all have good Perception

2. When building your party, include an Oblivion cleric

3. Very minor optimizations like making sure you never start traveling when it's close to dark. You don't want your characters to run 10 feet out into the wilderness and then pitch tents and get attacked in their sleep when you could have just stayed at the inn and left with 8 hours of daylight to use. But at best this is saving you 1 possible encounter, it's a minor and situational thing to watch out for

4. The big one you can do in gameplay is choose your travel speed. Slow, medium, or fast. As I discussed before, going Slow gives you advantage on your Perception check if and only if the game coin flips the encounter to happen during travel instead of camp. Going Fast gives you disadvantage on it if it happens during travel.

So at first glance, Slow sounds good. But there is a major downside, which is that Slow makes our journey take twice as many days as Fast. Which means twice as many encounters (roughly). Which means twice as many nights when we might get ambushed with going slowly not helping us at all. And for that matter, twice as many days when we do have advantage, but could still end up in the tail of the distribution with a bad roll, and then get ambushed anyway. If you have to do the slightly risky thing twice, the risk grows!

Which is the optimal strategy- fast, medium, or slow? Well that's actually just a solveable math problem, but the solution depends on several variables. Enemy perception, your perception, and... what the odds are of you getting party wiped. That depends on your party builds and your skill, but most of all it depends on whether you get surprised and whether the particular group of enemies happens to be one of the ridiculously deadly ones or one of the easy ones.

If you get surprised at level 3 by a Goblin Shaman + 3 dire wolves (which seems to be a very common encounter, showing up about 10 times out of 150 or so total results for me), it's almost certainly game over right there. 3 party members are immediately going to eat 42 damage from a lightning bolt, maybe one saving for 21. And at level 3, even your paladin probably just has 28 HP and will go down in one hit, so you're looking at half the team down before they get a turn. And that's before the direwolves chew on the survivors... and before it might turn out that the goblin won initiative and gets to blast away at whoever is left, etc.

If you get ambushed at level 3 by a Warlord +2 berserkers + 1 cult fanatic (another very common one in my results), the warlord alone could wipe out your entire team in one action surge turn with his 6 attacks if he has good luck.

But if you get surprised by something like 5 basic wolves? You're probably fine even at level 3.

My ballpark estimate, looking through my list of all encounters I faced, is that at level 3 if you get surprised there's about a 50% chance you immediately lose your ironman run. Even in a neutral encounter, it's maybe about 25%. That goblin shaman could still wipe you if he just wins initiative with his good dexterity and immediately fries 3 guys with his lightning bolt. If you win the surprise though, you should have a 100% chance of survival... because you can just choose to hide from these terrifying foes instead of fighting them at all.

Travelers' Tails

So how to decide whether to travel fast or slow? Well let's say for the sake of argument that your party is level 3 and has an average of +4 perception. That's quite feasible if they're mostly Sylvan elves and you've got a cleric or druid with good Wis, etc.

Let's call the enemy average perception +1.5. That's being slightly pessimistic, but I think it's reasonable for the low level Caer Cyflen region in particular, where a decent number of enemies are things like Scouts or Dire Wolves that have solid perception.

Let's also say my ballpark estimate is correct that being surprised at level 3 is a 50% chance of a party wipe, and even a normal encounter is a 25% chance of being wiped out (something like encountering a Necromancer + 3 Cult Fanatics + 4 Skeletons right outside Caer Cyflen is possible at level 3 and I can tell you from firsthand experience that it's a brutal encounter to try to tackle at level 3, as is Warlord + 2 Berserkers + Cult Fanatic)

Warning: the next few paragraphs contain graphic depictions of numbers being subjected to mathematical operations. Viewer discretion is advised. Numerical discretion not guaranteed.

!Some probability math (anydice can help. You can use a formula like this for if you have disadvantage: "output (4 + [lowest 1 of 2d20]) - (0+ 1d2 + d20)") can show that during the camp encounters the chance of your +4 perception beating their +1.5 by 5 and thus surprising them is a 40.5% chance that you're in the clear! 21.13% chance that they surprise you and thus you're 50-50 to live or die. Which leaves a 38.37% chance that it's neutral, and you have a 25% chance to be wiped out. !<

!So if you have a camp encounter, that's about a 20% overall chance that you're wiped out, which doesn't change with the travel speed.!<

!If you're going slowly and get a travel encounter, you have a 56.66% chance to surprise them and be safe. Nice! 9.18% chance to be surprised, so half that of being wiped out. And that leaves a 34.16% chance of a neutral encounter which has a 25% chance to end in disaster. So overall 13% of a travel encounter killing you when going slowly.!<

!What about for fast travel? Now you have disadvantage if you've got a travel encounter. You still have a 24.34% chance to surprise. But now a 33.07% chance that you ARE surprised. And a 42.59% chance that it's neutral. So the total chance of being wiped out is about 27.18.!<

!Next, remember that travel encounters vs camp encounters are 50-50. So the overall chance that a day of travel kills you for slow travel is the camp chance + travel chance divided by 2. (20%+13%)/2 = 16.5%!<

!And for fast travel it's (20%+27%)/2 = 23.5%!<

!So far slow travel looks better. But remember that you have to do twice as many days of slow travel, which means twice as many encounters and twice as many chances to roll low and end up in the tail of the distribution. For the sake of simplicity let's say that the fast travel guy here arrives after 1 cycle and the slow travel guy therefore needs 2. In that case the chance that the fast travel guy arrives alive is just (100%-23.5%)=76.5%!<

!But the chance of the slow travel guy arriving alive is (100%-16.5%)^2 = 70%!<

Uh oh.

Counterintuitively, fast-traveling is actually SAFER. Now the details can vary, this was just to demonstrate the basics. When you hit level 5 for example, your perception goes up (which makes it easier to avoid being ambushed) and I also think it's fair to say that your chance of being annihilated by an unfair encounter drops a lot once you have access to Hypnotic Pattern or Counterspell and so forth. But the general pattern holds. The gap closes, but does not really go away. If your assumptions look anything like mine, Fast Travel is safer than Slow Travel.

If your goal is to maximize getting your ironman run from point A to point B alive, your best bet is to fast travel and have a 76.5% chance to make it, vs only 70% if you slow travel with these plausible example numbers.

Psychologically this feels bad. It feels like the wrong choice to me. I do not want to click a button that looks like it says "Gamble your entire run on a 24.5% chance of losing!" But fundamentally, that's what happens with the Slow option too. You just have to survive twice as many luck-based checks to avoid being in the tail of the Perception distribution to see if your run is allowed to continue.

Let's get a quick example of how much an Oblivion cleric would help with these numbers.

!Your danger level when traveling is unchanged either way. But in camp, you no longer ever get surprised, so you have a 59.5% chance of a neutral encounter. And we're estimating that a quarter of those are fatal. So we've gone from a 20% or so chance of being wiped out in camp, to 15%.!<

!So with fast travel now the overall chance of a wipeout in however many days constitute enough time for 1 encounter is (15%+27%)/2 = 21%, so a 79% chance to survive.!<

!With slow travel it's (15%+13%)/2 = 14%. But then as before we need to bear in mind that the slow traveler has to survive twice as many encounters. (100%-14%)^2 = 74%!<

So fast travel survives 79% with an Oblivion cleric, and Slow travel survives 74%, meaning Fast travel is still better. Though the gap did close a bit.

And if you're playing the optimal strategy of doing Fast Travel, an Oblivion cleric is reducing your chance of losing the run from 24.5% to 21%.

Also remember that if your party is less optimized and has worse perception (or has some non-elves who must worry about sleep), the Oblivion Cleric gets even better.

As a rule of thumb, you can estimate that an Oblivion Cleric reduces your chance of losing to a random encounter by about 1/6. Pretty solid, I'd say!

There comes a point in the game (based on your party's power, the difficulty, and your own tolerance for risk) when random encounters switch from being a lethal threat to being a source of resources: XP and gold. At that point, it makes sense to switch to Slow travel.

But if you're trying to get an Ironman run to the Dark Castle intact?

Fast and reckless wins the race.