The gas smells awful, but there are no ill effects (to Thane). It seems to be... thickening, if that can be possible. Striations of purple and black energy and small bolts of crackling green light are now rippling through the gas cloud like a small thunderstorm.
Something seems to be coalescing in the center of the gas cloud. A flash of shimmering eel-like flesh glides just outside of the cloud for a moment. Whatever it is, it's big. A serpentine creature as thick around as a man's waist.
Thane tries the door- it is locked. He hears a muffled chuckle on the other side. "...You'll never take me alive, devils!" Followed by some chanting of magical syllables on the other side of the door.