Who knew that wraiths are rigid creatures, being in same class as DM’s, crabs, crystal guardians. As such, they would take extra extra damage from a Duelist’s pickaxe.
On the same time, Mimics are not rigid (they’re demonic). They’re one of the very few demonic creatures that can appear in sewers.
I suppose it make some sense they have some corporeal features though, how else could you attack one with regular weapons, or equip your sad ghost with a weapon and armor?