From Pathfinder #186: Ghost King's Rage, which isn't enabled.

Compression Tunnel

COMPLEX HAZARD 20

Stealth +41 (legendary)

Description This shadow tunnel compresses when creatures pass through.

Disable DC 46 Athletics (legendary) twice to push back the shadows or DC 48 Thievery (legendary) three times to pierce through the shadows

Shudder

Trigger A creature reaches the tunnel's midpoint

Effect The tunnel shudders, knocking creatures Prone unless they succeed at a DC 42 Reflex save. The trap then rolls initiative.

Routine (1 action) The tunnel compresses, crushing all creatures in it and dealing 4d10+22 bludgeoning damage (DC 42 basic Fortitude). On a failed save, a creature additionally becomes Immobilized (Escape).

Pathfinder #186: Ghost King's RageOGL