Can I ready the spell Feign Death with the trigger of being hit?

I feel the need to mention that the spell contingency exists, which is much more effective for this purpose. It will trigger automatically on a pre-specified condition, and doesn't require you to be actively casting a spell, that the players can all see him casting, holding, and releasing.

I'd say yes, concentration check.

Also, are you sure that's a good idea? The spell lasts for an hour. I mean, I don't know your players, but I know quite a few adventurers would make sure the guy's dead by separating the head from the body or just burning him entirely. You know, to prevent ressurection. Perhaps he should Dimension Door away instead? Or just, you know, make a tactical retreat.

Oh, and you are aware he can't end the spell himself, yes? It takes an action to dismiss, and you can't take actions while incapacitated, so once he casts it on himself it's full duration or dispel magic.

Assuming he is fighting the party he'd still have to cast the spell in front of them, unless he breaks line of sight or something, and if he casts a spell that does nothing the players may be suspicious. As other people have said contingency is a way around this, or just give him a magic item that casts feign death at a certain hp threshold. Extremely niche if the players get it, but perfect for your purposes if you're set on having him play dead.

1. yes

2. sure, youll still need to make a concentration check

3. you could set the trigger so that it could be released before youre hit, but that would probably lower whatever DC the players would have to beat to catch on to this

just be prepared for this to not work more than once. hell, it could possibly not work once if the players just decide to do something weird like decapitate them (you can never know with players...)

Seems other people answered it. But even if was a no it does not work that way, ss the DM you can make up powers for your BBEG.

As long as you do not go over the top do what you want. It is the BBEG, give him cool shit to make it more epic.

The spell does have components, so you'd have to hide those somehow

Yes that works (as people have already said) but tbh there's probably a 90% chance your BBEG ends up dead anyway. If this guy's best chance of escape is casting feign death, he doesn't sound like much of a BBEG. I'd recommend a higher level spell, or better yet just make something up. Villains don't have to be NPCs. They can be like monsters, and monsters can do all kinds of things that the players can't.

This BBEG needs a lot more in their arsenal that just Feign Death. If you’re planning for this to work, expect it not to. Either the CON check fails (or you are forced to fudge it), the PCs proceed to stab the BBEG on the ground or do something random like cut of his head (because players are random sometimes..)

If you want this guy gone, have him gone. Dimension Door, Planeshift... The Contingency Spell is very good for a prepared triggered spell. Honestly though, if you don’t want them to die, don’t put them in front of the party.

Contingency spell.

With the trigger, If hit or damaged -> Activate Feign death.

1, can you touch yourself? If yes, then yes, you xan cast it on yourself.

2, yes. Ready action "cast spell", trigger "when I get hit, I cast feign death." You will need to make a concentration check when you do get hit though.

3, if the trigger is "when I get hit," you have to be hit for the trigger, otherwise the ready action won't go off. If your trigger is "when I am attacked," you could do it when your attacked...but if it hits, concentration check. If it misses, it's not convincing because...they didn't get hit.

Or you could just handwave it, have him have a modified ring of spell storing that allows the user to set a condition for an auto cast (like the shield guardian's function) or best yet, if it's his lair, have him lay out a glyph of warding in a certain spot, with a trigger like "when a creature stands here and says 'no, I want to live!,' release the spell on that creature" and store feign death in it.

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Personally I'd allow it to be without concentration for that same hit.

But I'd also have a deception check used to determine if they were successful

You can hold an action to cast. You trigger held actions off of an event you can perceive. If the action is being hit or after being attacked, your action triggers on the hit and you cast the spell with no need for concentration check as you cast it post hit.

Now, technically, you cast the spell on your turn and hold the use for your trigger, losing it if it didn't happen before your next turn. Personally, I think that's kind of bullshit as you can cast and hold non concentration spells. If the casting time is 1 action, and you hold your action, you've not used your action yet. Holding a fireball in your brain waiting for the moment is silly. So it's a house rule that I do that a held action is a held action.

"When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration. If your concentration is broken, the spell dissipates without taking effect"

I don't like that idea, losing spell slots sucks, and (again) don't agree with the concept of holding massively explosive spells in concentration.

The problem is unless you give the metamagic ability of subtle spell, he will be talking, holding materials, and weaving his hands around casting and it'll be pretty obvious he just did something funny.

Honestly, if your want him to live, give him something that let's him do so. You don't need to pull punches with players. He sees it going south so he uses a reaction for misty escape and teleports out. NPCs don't follow PC rules, it isn't cheating to make him whatever you want. Make him a doppelganger or a simulacrum or whatever. Just flat out keep him alive or they might burn his body out any number of crazy things players do. Leaving a body with players is not a safety net

The big problem I can see is that it won't look like they were killed by the attack - it'll look like they got hit, and then died of a heart attack.

Pitching the narration of this right to ensure that it doesn't feel unfair will be a challenge, and the monster should absolutely make a perception check.

The fact that the BBEG is dying instantly at 0, rather than bleeding out might also be a clue, depending on how you run your game.

Death Ward, plus contingency to cast Feign Death when the Death Ward drops, may be more convincing.

I'd probably have a henchman do this, then have the BBEG use a more powerful equivalent later in the campaign (eg contingency, set up to teleport a small part of their body away on death (a custom spell, but one the players can find development notes for), onto a glyph of warding set to cast Reincarnation into any bit of flesh on top of it.)