Wraith

The Wraiths are Hardmode ghost-like enemies that can travel through blocks. They can inflict the Slow debuff (which makes you go slower), and have glowing red eyes that are visible even in complete darkness. They spawn at the surface layer at night.

Wraiths are generally one of the primary reasons why most earlier AFK surface farming techniques failed once hardmode came around - by floating through barricades and ignoring lava, they, along with pixies make lava pits alone incapable of eliminating all enemies, and will kill a player who AFKs. Wraiths are immune to lava and do not trip any pressure plate, but can still be harmed by traps that are triggered by other means, such as timers or other mobs setting them off.

Wraiths appear to be flying, however within empty space they can only hover slightly off the ground. They can only follow the player upwards when in blocks. A more accurate description would be that they "swim" through land.

Tips

 * Wraiths' inability to fly upwards in empty space can be exploited when building your Town, by ensuring there is adequate clear area beneath it (essentially "floating houses"), which protects against Wraiths during Blood Moons. Given enough clearance off the ground to avoid jumping enemies, floating houses can also protect against other enemies and Goblin Invasions.
 * This trick also applies to fighting wraiths - a platform raised at least 7 tiles above the ground prevents wraiths from reaching the player, letting the player kill them at his or her leisure.
 * Wraith's cannot pass pearlstone blocks (tested on xbox360 version)

Trivia
Judging from the armor of the wraiths they maybe are ghosts wearing Ancient Cobalt armor.