Wraiths are Hardmode ghost-like enemies who can travel through blocks and 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.