[Campaign] - Elemental - Napoleonic Era quest to stop the balnce of the elements being destroyed

In our group, we have rotating GMs with pretty much everyone in the group taking a turn at some point. On that basis - if you are in the 'Event Horizon' group, please do not read. Anyway, it will shortly be my first turn so thought I'd lay out my ideas for a game! This also turned out a lot longer description than I expected...

The world is composed of 7 planes - the four 'elemental' planes (fire, earth, air, and water) and two... 'energy' planes (light and dark). Where these planes intersect, we get the normal 'material' world. The balance between these planes is unstable, meaning that some areas of the world have higher levels of 'fire energy' or whatever and this causes volcanoes to be more common etc. The influence of the planes sometimes also affects creatures allowing them to have limited control over that element - this is the only form of magic available. Also, humans are the only sentient race on the material plane.

The secret history not known to the players or any of the humans: In the past, the balance between the planes was extremely unstable and the world would suffer from frequent elemental storms which would cause massive earthquakes, hurricanes, monsoons etc. Over time, this was discovered by scholars and in order to limit its effects, a network of 'keystones' was built around the world which helped stabilise the balance in that area. As a side effect, these also reduced the amount of control that 'magic users' could have over their element - as well as descriptions of these elemental storms, the old legends from ancient kingdoms contain descriptions of feats which are thought to be impossible. For some reason, the knowledge of these keystones was lost (kept secret, lost in wars, plagues, etc. - not sure yet).

Currently: The current state of affairs between the human nations is pretty much England v Napoleonic France/Europe. The tech level is muskets and rifles, with square rigged ships, etc.

There is a cold war between the inhabitants of the plane of light and the plane of darkness, with the normal material plane caught in the middle. Due to the way the planes work, creatures can only live on their own plane, but can survive for a time on a plane 'next to' their native plane - i.e. humans can survive on both the light and dark planes, but light creatures would die quickly if they entered the dark plane and vice versa. This means that they can only really act against each other using humans as proxies. A faction within the lightside wishes to end the stalemate and get on with fighting, and has found a way to modify the keystones to make their plane dominant in an area of the material plane. Due to being limited in the actions they can perform, they realy need humans to make these changes for them though, at least until the balance has been adjusted enough that they can come through en masse.

This lightside faction has successfully created agents provocateurs by tricking a group of people into forming a darkside cult. They have given them the locations of one of the keystones (located in hidden caverns under a major building in a major city) and told them to go destroy it for the glory of eternal darkness, etc. The lightside factions current plan is:

1. Get the fake darkside cult to smash one of these stones, causing mass chaos and devstation as the elemental effects in the area snap back into their original balance.
2. Human nations now strongly opposed to darkside and invite lightside in to assist.
3. Lightside tells human nations 'oh we could help so much more if only we had more influence in this area'
4. Humans modify keystones to allow lightside in.
5. Lightside enslaves humanity and uses them to attack the plane of darkness directly.

The quest for the group I am expecting to go something along the lines of:

1. Respond to attack by cult, finding keystone in process. Either successfully defend it, or not.
2. Research into keystone and what it does. Would be some maps, information there allowing them to get a few locations of others.
3. Probably get sent off to find some of the other nearest stones, and make sure they are protected.
4. Maybe assist into modifying some of the stones to allow lightside access.
5. Realise they have been played and then start fighting back against the lightside invasion.
6. Probably end up rebuilding the keystones which have been broken/modified.

I am planning to have one of the sides (probably the fake Napoleon side) in the human national combat ally themselves firmly with the lightsiders.

TL, DR; Hornblower + Babylon 5 + people throwing fireballs at each other.