Ghoulder

It is a boulder trap with a haunting attached. Knock out its support and instead of rolling once and stopping, it becomes a ghostly version that follows whoever is closest. Point your character at it and it drops out of the air and sits still. Look away and it starts moving again.

Since your character faces the cursor, the counter is to keep the cursor on the far side of you and back away rather than turning and running. A wide dungeon corridor is the dangerous case, because it has room to build up a chase behind you. In a tight tunnel you can freeze it with a glance, walk past, and leave it stranded.

They generate in the ground around the Dungeon, so you meet them while tunneling in rather than inside the building. The rule about it only moving when unobserved is a Boo joke.