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 with having to leave some encounter for later (had same situation in the Nephar Fortress), non-linear questing/exploration is so much better than the linear AAA stuff; but you should be able get out (as long as you're out of combat and not obviously trapped). Or maybe I should just sometime learn to keep saves in addition to the most recent quick save, after playing cRPGs for almost 30 years...
Anyways, love this RPG, esp. the story, lore and world building. Blows my mind it released years before BG1! Very impressive.