We're inside the giant obsidian skull: Vecna's fortress, the Nevervault. "How exhausting it must be to hope," Vecna tells us, and the doors seal behind us.
Vecna stands at the far end of a corridor of giant glass cylinders, a pulse like a heartbeat whispering through them. Four skeletons stand around the room, and on closer inspection they look familiar: Goblin Laura, Gnarls' brother, one of Jain's Aspect Warrior sisters. Fell doesn't get a reveal; he already killed his went we entered the room.
The deeper we go, the stronger the feeling of holding secrets that aren't ours grows. An armoured knight swings out of the vats, and Jain smashes it, and then three more arrive: two Whispers of Iron and a Whisper of Steel. Gnarls knows the last one. Lord Soth, the death knight, a creature of legend. He doesn't belong here, and that's the point: Vecna is imposing his own reality on the multiverse, pulling others into this one and warping it to fit.
Gnarls casts true polymorph on himself and becomes a beholder. His central eye throws an antimagic cone down the corridor, and the Vecna at the far end simply vanishes: an illusion all along. Soth crashes into Gnarls and Sugar and knocks them flat, Fell lands a huge hit and survives a Word of Death, and the knights finally fall. Vecna's voice echoes after us. We're only delaying the inevitable.
Up the stairs, strange alternate realities phase in and out, and we lose threads of time. We come out into a huge liminal space, all floating purple dream-stuff, with a wall dividing it and an enormous pile of bones at the top. Behind it is Vecna, the real one, very big, working his ritual in time with the glow. The Hand of Vecna speaks to him, and he rejects it. The Hand is furious.
One of the bone piles explodes under us. We push forward, and Vecna teleports into a smaller pile behind us, healing massively every time he does. The piles seem to give him power, or a way to move; ours is gone, since nothing magical will carry us here.
The party is whittled down, and it's looking bleak.