High-level poison spell

[Vaamat]

As a preface, I'm aware that poison magic is intended to be weak in the late game, and I don't mean that to change. Still, within those constraints, it might be fun to have a more exciting top end for poison mages. This might be achievable if these spells are pitched toward very niche, underpowered character builds and if they remain ineffective against poison-immune monsters.

So, what about

Pestilent Influence (level 8 Poison/Hexes, appearing in Book of Alchemy): Silently create darkness and miasma clouds at random in a large region. Does not alert enemies, although they wake up if they take damage.

The primary use case would be for stabbers, who could conceivably use the clouds for concealment and evasion while weakening crowds. I could be wrong, but I don't think stabbers with high Poison Magic are really a thing right now (despite being a flavorful combination)? Stabbers usually want a more reliable "plan B," and curare makes poison a little redundant.

Another major use would be for Vaults:5, where it seems pretty strong. But stabbers need all the help they can get on Vaults:5. Meanwhile poison magic is already pretty strong there (especially with Ignite Poison, which is in the same spellbook). Non-stabber venom mages may not need to invest all this extra skill in order to beat Vaults:5, but if they do invest it, the reward seems fair.

Another use is to block LOS, but I imagine that could be balanced. It seems like a lot of effort for a mega-Summon Butterflies. A minor use would be the combo with Alistair's Intoxication, which is in the same spellbook. The combination would greatly weaken enemies and make them more likely to stumble into miasma clouds, but it's using a ton of mana and spell slots and is still no use against nonliving enemies.

In general the biggest beneficiaries I can think of would be nagas, formicids, and DEEns, none of which seem to be in danger of becoming overpowered.

[Khashishi]

By the time you can cast a level 8 spell (especially a dual school level 8 spell), you are already tackling the extended end game, where most enemies are immune to poison. So, it sounds like the main point of the spell is to be a more powerful scroll of fog. Fogs are very useful, though probably not worth 8 spell points (though that depends on what you mean by "large area"). It sounds to me more like an air spell though than a hex.

[chequers]

The combination of los blocking and damaging clouds from the same spell is tough, because damaging clouds dissipate extra quickly when out of LoS. Usually within 0-2 turns

[petercordia]

It's actually transparent clouds which dissipate out of LOS. Steam (currently the only LOS-blocking damaging cloud type) is immune to this.

[svendre]

The fix for the poison magic school is to stop nerfing poison arrow and make it easier to find in more spellbooks. These were both changes that made the school very awkward to play.