Idea: Ghosts
They exist in darkness level 0, at a specific underground level.
They tend to let out moans that are similar to cave sounds in some way.
They have the ability to go through most solids, each solid having a limit to how many blocks it can travel through.
Lets say sand, and dirt allows them to travel through 4 blocks.
Cobble and smooth allows them to travel through 3.
Iron is 2.
Gold would be 1.
And obsidian is 0.
They cannot harm the player in lit areas, and disappear ( Or become more and more transparent when closer to light source) when any source of light is presented.
Would do significant damage to player lets say 4 hearts. (Edit: Some ghosts are not harmful and seek your help to repair a gravestone = gold apple or some treasure.)
But the player cannot hurt them without a specific weapon.
Instead of a dungeon spawn, they appear one by one, or even in pares of two around tomb shaped stones that must be rebuilt or destroyed.
This would make the Minecraft experience a bit more frightening when torch light sources will become temporary.
Poll