knife that won't cut

Exploration Stage: Manor House Gate
From: Huntress
Description: I think I know what it is that interfered with Martha's divination. But it's still too early to conclude. I saw a knife in the ground at the gate. Let's find it. If we can find it, we can figure out what the monster was that came during the divination.
Completed: See. This knife is a technique used in very old magic. This monster changes its appearance by doing somersaults on a knife stuck in the ground.