Wraith

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

Wraiths appear to be flying, however within empty space they can only hover slightly off the ground. They can only fly upwards when they have foreground blocks to "swim" through.

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 a "floating house"), 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.
 * A platform raised at least 7 tiles above the ground prevents wraiths from reaching the player.
 * Wraiths, along with pixies, make lava pits alone incapable of eliminating all enemies. They can still be harmed by traps that are triggered by other means, such as timers.

Trivia

 * Judging from their armor, they could be ghosts wearing Ancient Cobalt armor, though the helmet is different.