One of the family of step-and-shoot traps: step in the wrong spot, and something shoots out at you, in this case a spear.
Detect: Per-based Traps at -2.
Disarm: DX-based Traps at -2.
Circumvent: Automatic (don't trigger).
Evade: Hearing at -1 allows Dodge at -1.
Effects: 1d+1 impaling.