Thanks for the reply! I'm playing on normal difficulty.
It seems you can't get out of there without killing Drath, at least with my tool use skill (there is one switch I can't operate, so maybe that might open an exit?).
Well, I lowered the difficulty to casual and then could bring evil Drath down to the point where he's fleeing ("Drath takes 19 points of damage. (163 blocked)" LOL). Kind of a bummer, but I'll just declare it a bug that you can get stuck down there, so the blame's on Mr. Vogel (*shakes pitch fork*) and I can continue with my life :-)
FWIW: I'm fine