From The Heretic Knowledge Vault
We're not sure exactly where or what this thing is, but at least it's what the title says: the place where Senilis is sleeping, at least until a bunch of elves (not to mention Jon, Sara and Meji) come around to disturb his slumber. It seems to play the same role for Senilis as the Giant Magic Potato does for Anilis, or did until Ian came along. It's first seen here, and becomes the venue for quite a bit of subsequent action.
In contrast to Anilis' digs, which had a rather vegetable feel to them, this place gives the impression of a giant underground chamber that was constructed rather than grown. It's crisscrossed with a network of paths and catwalks hanging in the air, with a notable absence of handrails or other things to keep people from falling off them to a messy end. An unnamed elf, in fact, has exactly this happen to him/her as a result of being popped in the chops by Sara during their escape from the chamber. We don't know whether the elf has reached bottom yet, but it's going to be ugly when he/she does.
When first encountered, insane Paedagogusi Concussion is hanging around on one of the catwalks, and gives nasty elven High Commander Yenhael a frontal lobotomy to "help" him find Senilis. Fellow Paedagogusi Chicanery, Nookie and Kawaii show up shortly afterward; Continuity hasn't put in an appearance yet. Senilis himself (the "ol' bastard" to the Paedagogusi) was in a receptacle of sorts somewhere in the middle of this thing until rousted by the elves. Where he is now, though ... interesting question.