Ghost

Ghosts are spirits of the dead, probably fallen adventures who died in these caves. They are easier to see in the dark than in light. Be warned, normal weapons and attacks are useless against these intangible beings, so you better have something magical if you want to take one down.

Origin

It is a common trope in RPGs that certain creatures can only be harmed by magic or magical weapons -- and a reasonable one for creatures without physical bodies. The ghost was added specifically to embody that trope.