The 12th-floor abode of the pantheon — a realm of bizarre gods that the fifth-floor deity storyline unexpectedly reaches toward.
The Celestial Ascendency is named in the possessed doll head's system entry as the twelfth-floor halls where the pantheon of true gods dwells — a place of 'serious whackadoodle business' (a god birthing a snake-headed fortune-telling cow from his thigh, a woman with sentient menstrual blood). It's the rarefied club Psamathe, a mere banished minor deity, was cast out of. Mordecai is astonished the fifth floor's quest invokes both Larracos and the Ascendency, a sign of unusually deep cross-floor storytelling — and a hint of the strange divine tiers awaiting far below.
Not directly seen; the twelfth-floor halls of the gods, evoked only through lurid secondhand descriptions.
The distant twelfth-floor seat of the pantheon, looming behind the fifth floor's god-storyline.
Also appears in books you've already read — spoiler-safe to their endings: