I know this is coming out of left field, but take a look at the back of the DMG 1E where it allows you to generate "alternate appearance" demons.
I'd use regular ones, but make them appear bizarre and twisted. As for other manuals, the Demons & Devils stuff from Erik Mona has lots of 'monsters'
..and throw in a little "Lankmar" just for flavor..but twist it. Familiar but creepy as hell can give really put players on edge. Can you imagine having an NPC they'd encountered there, having been thrust into the Abyss as part of his death punishment (instead of Heaven) and been mutilated and bearing such a hatred for the PC's for 'offing him'..but he's got a tentacle coming out of his eye now and sloppy slime appendages? Think the movie "The THING."
Just because it's on another plane doesn't mean that you can't have a connection to the material or prior adventures.
jh