Wraith

Wraiths are Hardmode ghost-like enemies that can travel through blocks. When a Wraith attacks, it occasionally inflicts the Slow debuff. Wraiths have glowing red eyes that are visible even in complete darkness. They spawn at the surface at night, and underground at all times.

Wraiths appear to be flying, however they can only actually "fly" upwards when traveling through blocks. They must essentially "walk" along the surface otherwise, and can not fly upwards through empty air.