Beyond the Black Curtain

After breaking their sworn oaths in a fit of forbidden passion, a sacrificial bride (Shekalane) and her fearsome escort (the ferryman Dravidian) find themselves alone and on the run in the subterranean river-world of Ursathrax.

From the book:

Permission would not have been granted, nor did he ask; instead, he went straight to the detention block after his meeting with the prefect and located Shekalane's cell. It was easy to do, for it was the only one with a light beneath its door. Indeed, i...