Wraith

Wraiths are dark ghost-like enemies with glowing red eyes which only spawn in Hardmode. They have 200 health and can travel through blocks and walls. Wraiths are a severe hazard to the liveliness of your NPCs due to their ability to pass through blocks and walls. When not inside any blocks, Wraiths hover over the surface and can not gain any height. This can be exploited when building your home by ensuring that the area Wraiths can reach along the ground floor is not occupied by any NPCs.

When a Wraith attacks, it occasionally gives the Slow debuff.

One or two Wraiths will immediately spawn after destroying a Demon Altar.

Wraiths are more common during a new moon.

Wraiths can be attacked through walls with melee weapons or other weapons whose attack animation does not end when it hits a wall - this makes spears especially effective against wraiths, as you can attack them through or while they are inside of a wall.

The Wraith could be a reference to the "His Dark Materials" series, in which wraiths are summoned whenever the subtle knife is used to open a portal