Our levels are already maxed at this point. So, I buff for the battle. Protection from evil 10ft. radius, remove fear, and Jah (who is going to tank off against the demon while the rest of the team takes on the cultists) gets every potion I can throw at her (heroism, mirror eyes, storm giant strength, fire resistance, etc). Also, I throw a free action on Minsc and jah, just for funsies.
I get into the basement, the dialog triggers, and I send out a cloudkill to get the cultists in the back when the demon moves to engage Jah. Minsc is working on the main cult wizard lady, and by the end of round one, half the team is held. In round two, the cultists (even the ones outside the cloudkill) all explode and turn into demons. By the end of the round, only Minsc is unfrozen, but he's about 20hp from dead (and the next round ends him).
So, for starters, I'm thinking the cloudkill was a bad idea somehow, but I would think that would only have effected the mobs that were in it. Aside from that, I can't figure out how we all got wiped out so quickly.
Side note, does -everyone- need mirror eyes? I figured it was just whoever was engaged with the demon. However, that wouldn't explain why Jah was held.
Anyway, I'm not sure what I did wrong here...any ideas?