Wraith

The Wraith is a mob that was introduced in the 1.1 update. This mob is not solid and can therefore pass through blocks. They will appear after Wall of Flesh is defeated.

They will also be summoned upon the destruction of a Demon Altar. The Wraith, like other creatures able to pass through blocks, can be damaged through walls with any melee weapon, as well as by spells such as the Vilethorn while they are still inside a wall.

When Wraiths are not passing through blocks they are unable to hover above about 3 blocks in height. Keep your NPCs above that height to ensure that Wraiths don't kill them.

Update Info
1.1 - Added to the game