I was wandering around the prison level when I came across a couple of drow robbers. I finished them off with my newly acquired sword (you know the one I'm talking about... BOOYA!)
Then, something peculiar happened : three of my characters started suffering from Cloudkill effects. I tried "Neutralize Poison" but it didn't work. Luckily my cleric was OK so I secluded the group of 3 that had the plague and fireballed them until they were toast. I them rased them and rested and everything was fine.
Has this ever happened to anyone before or did I stumble on a trap that I didn't notice?
I seriously doubt it was a trap because the only things on the game that have traps on them are doors and chests. Unlike BG there aren't traps just on the floor, so it's entirely possible that you ran into a bug of somesort.
"I'll take the stupid one who decided to threaten us, instead of shoot us when he had the chance" - Bao-Dur
In the Lower Keep there are Traprunes that will detonate a fireball if you over it. I thought that was what I may have encountered.
What's the deal with the green, glowing wave of energy that emanated from the tomb with The Draedsweord Daemoniac in it. Is that harmful? The Cloudkill effects started to happen as soon as I left the area.
If it is a bug, it's a particularly nasty one! If anyone else has this happen to you, pray it does happen to your cleric!
Actually the two gates that block your way to The Draedsweord Deamoniac are heavily trapped, the first one has a cloudkill trap and the second one a fireball. Maybe something buggie happened and the trap was triggered after you opened the gates.
And the sarcophagus there seems to cast disrupt undead but since the undead there are powerfull creatures they are immune to it.
"When I am working on a problem I never think about beauty. I only think about how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong." Buckminster Fuller