Don't worry, That's not Rei that was strung up.
And you were curious as to what that giant creature was in the Xenos chamber. It was a dragon. In the light novel, they explain that he is the eldest of all Xenos, but is so old now that he can barely move.
Brandon Sabin
2021-02-18 03:32:04 +0000 UTC
Fels' voice is technically supposed to be androgynous, and altho the pronoun "he" is used for literature reasons, technically nobody knows their gender from a factual POV, even in-universe or outside-universe
Also, since Ouranos' prayers are the only thing keeping the Dungeon relatively stable (so no monster rushes to the outside in the last millennium or so he's been on Gekai), there's a pretty popular fantheory that on the bottom-most Floor, the "last boss" is probably Gaia, which makes sense thematics wise