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 follow the player upwards when in blocks. A more accurate description would be that they "swim" through land.

As of 1.2 wraiths also have a chance of dropping a mechanical eye. The percentage of it dropping it is unknown.

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.