Cleansing of the Cenotaph
Marching on, there is another stair, and it goes deep. Very deep. Other levels branch off occasionally, stretching out into the dark, but they are mostly empty, with only one containing a legion of skeletons - hundreds, perhaps thousands - standing stock still as though awaiting orders. That One, without prompting, annihilates them with its breath in waves.
Finally, the stair comes to an end. At a guess, you're near the bottom of the canyon now, and it opens onto a lavishly decorated floor - mosaics in the floors, chandeliers of magical lights hanging from the ceiling. Moving forward, you pass through empty chambers until you reach what looks like a ballroom - complete with dancing couples. Each is a skeleton, finely dressed, moving with tireless, mechanical grace to the same music, though you hear nothing. At the far end is a curtained doorway into another room.