Wraiths are Hardmode ghost-like enemies who can travel through blocks. They can inflict the Slowdebuff on the player. They have black bodies, which render them almost completely invisible in dark areas, but have two glowing red eyes which are visible from within the darkness. They spawn at the surface layer at night.
Although Wraiths appear to fly freely, they can only hover slightly above the ground. They need foreground blocks to "swim" through in order to fly any higher.
Wraiths can be attacked through walls with melee weapons or any other weapon whose attack animation does not end when it hits a wall. This makes spears especially effective.
Wraiths will not trigger pressure plates, as they pass through them instead. This makes them incapable of activating any sort of trap.
Tips
Any structure built at least 6 tiles above the surface will prevent a Wraith from levitating up and into it.
Wraiths are most likely a reference to the fantasy novels series The Riftwar Saga by American writer Raymond E. Feist, the first series in The Riftwar Cycle. There, wraiths are described as black man-shaped clouds with burning red eyes, which is further referenced by the fact that they rely upon their target being too scared to move, similar to how they can inflict the Slow debuff, hindering player escape.