Lunatic Cultist

The Lunatic Cultist, alternatively known as the Ancient Cultist, is a Hardmode, post-Golem boss summoned by killing the Lunatic Devotes and Cultist Archers that spawn at the Dungeon's entrance after Golem is defeated. Beating the Lunatic Cultist initiates the Lunar Events.

Behavior
The Lunatic Cultist starts off by warping to a new location and then unleashing one of three attacks: a burst of three homing fire balls which explode on impact, an ice attack which emits ice shards in six directions as it flies, and an energy orb which fires large bolts of lightning at the player. This is the Lunatic Cultist's direct form of attack. As the Lunatic Cultist becomes damaged, he will begin attacking with two spreads of five destructible ancient light projectiles as part of his previous cycle.

Occasionally, the Lunatic Cultist will attempt to spawn illusionary duplicates of himself. Unless the summoning ritual is interrupted by attacking the Lunatic Cultist and not one of the illusions, these phantom duplicates will stay, flying in formation and adding their homing shadowflame attacks to the Lunatic Cultist's. This will also result in the summoning of a Wyvern-like Phantasm Dragon, and if there is already a Phantasm Dragon inbound, an Ancient Vision, which looks like a miniature version of the Moon Lord's head. These phantom duplicates can be eliminated by interrupting the ritual again, but if not, the Lunatic Cultist can amass a large quantity of phantoms, making the fight much more difficult.

The Lunatic Cultist will go through an attack cycle between each summoning ritual, performing four or five attacks before proceeding. This cycle is repeated throughout the battle, and can become chaotic unless the Lunatic Cultist's summoning rituals are properly interrupted.

Tips

 * Avoid defeating the Lunatic Cultist if you are not yet ready for the Lunar Events, as after beating the Cultist the Lunar Events begin immediately, and there is no way to de-summon them (aside from defeating them). Take care that any minions/NPC's in the area don't kill the cultist in defending you/themselves.
 * Shooting the decoy cultist is not a bad idea if you have Vampire Knives. The Phantasm Dragon is very long and you can easily regain some life by hitting the Dragon with a wide barrage of knives.

Trivia

 * It is a common belief that the Cultist's design is inspired by Crowno's Developer Armor. However, according to a profile post of his in the forums, the Cultists were designed before it.
 * The boss' appearence resembles a plague doctor costume, which was a garment worn by physicians who treated the Black Plague in 14th century Europe.
 * It is the smallest boss, and the only boss without an awakening message, and the only character in the game that speaks (it says "How do you know" when summoned).
 * The word lunatic comes from the Latin luna, because it was believed that people were more likely to exhibit aberrant behavior during a full moon. This may explain why the Lunatic Cultist takes a part in the summoning of the Lunar Events.
 * The Lunatic Cultist seems to drop the "Ancient Cultist Trophy" rather than the expected "Lunatic Cultist Trophy." Likewise, it drops the "Ancient Cultist Mask" instead of a "Lunatic Cultist Mask."
 * The boss' attacks (using ice, fire and lightning attacks as well as summoning monsters) and overall fight is quite similar to numerous boss fights from the CastleVania series, such as Barlowe or Shaft.
 * If Skeletron isn't defeated in a world but Plantera and Golem are, the it is possible that the cultists won't spawn making it impossible to spawn the lunatic cultist.

History
Cultiste Lunatique Lunar Cultist