Wraith

Wraiths are Hardmode ghost-like enemies who can travel through blocks. They can inflict the Slow debuff 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.

Tips

 * Any structure built at least 6 tiles above the surface will prevent a Wraith from levitating up and into it.
 * Wraiths can be spawned pre-Hardmode by using a Wraith Statue.

Trivia

 * According to the Collector's Edition trading card, those who are devoured by an Eater of Souls become a Wraith. This might not be the only way of creating wraiths, since they are not exclusive to worlds with The Corruption, and will still spawn in Crimson or purified worlds.
 * According to the Bestiary, wraiths are souls belonging to the darkest of hearts. They are sealed in the world's core, the Wall of Flesh, until it is defeated.

History
Spectre 幻灵