Eidolist

The Eidolist is a Hardmode enemy that appears in the Dungeon as well as the 3rd layer of the Abyss.

Eidolists will stay in one place until they are damaged, in which they will attempt to fly towards the player, going through blocks and teleporting periodically. Once provoked, they will release lightning orbs and ice balls that emit ice shards. Both attacks are very similar to those of the Lunatic Cultist.

In Revengeance Mode, the lightning inflicts the Electrified debuff for 2 seconds, and the ice ball and ice shards inflict Frozen for 1 second, Chilled for 2 seconds, and Frostburn for 4 seconds.

Trivia

 * Judging by their appearance and behavior, they seem to be a Cultist partially transformed into an Eidolon Wyrm.
 * This is supported by the fact that Eidolon Wyrms also drop the Cultist set and cast spells, indicating that they all may have been Cultists at some point.
 * Despite being an enemy that spawns in the Abyss, it is unable to drop the Halibut Cannon.
 * The Eidolist still resembles an old sprite of the Eidolon Wyrm, which was heavily based off of the Ghost Leviathan from the aquatic exploration game, Subnautica.