Moon Lord

"Impending doom approaches..."

The Moon Lord is a Hardmode, post-Golem boss summoned by defeating all four Celestial Towers from the Lunar Events, spawning about one minute after the fourth Tower is destroyed. It serves as Terraria's "final boss".

If all players die during the Moon Lord encounter, the Moon Lord despawns. It can only be re-summoned by completing the Lunar Events again (initiated by again defeating the Cultists at the Dungeon's entrance, then defeating the Lunatic Cultist again).

The Moon Lord has two hands and a head that must be killed before its "core" becomes available for attack. Each hand, as well as its head, contain a vulnerable eye-like target area that are each periodically closed off and opened via an eyelid. These may only be damaged during their "open" states. Once these parts and then its core are killed, the boss is defeated.

During the fight, the Moon Lord positions its two hands on either side of the player some distance away. Its mouth periodically attaches a tentacle to the player that extracts multiple killable essence-like monsters out of the player. If not killed by the time they reach the mouth, the Moon Lord's Head is healed for 1000 health for each of them. This tentacle will also inflict the Moon Bite debuff, preventing any healing from weapons/armor like Vampire Knives or Spectre Armor for 17 seconds.

Tips

 * The Moon Lord is very slow and will not despawn, even if the player uses a Magic Mirror or other such item. While healing effects may be disabled for the duration of the battle, quick enough players can retreat to regenerate health and even resupply with their town NPCs.
 * The Moon Bite debuff, which prevents regenerative abilities, is reliant on a tentacle making contact with the player. It is possible to move outside of its maximum range, avoiding the debuff and allowing effects to once again heal the player. One such method is by using asphalt.
 * It is best to start by damaging the two hands until they are both at very low health. Next, kill the head. As soon as you kill the head, kill the two near-death hands. This will cause the eyes to come out of the hands at the second half of the fight, increasing your survivability during the first half.
 * If all players are killed during the battle, the entire event must be repeated to spawn him again.
 * Since the hands will begin to start dealing melee damage after they are defeated, running away on an asphalt sky bridge will become an issue if not properly prepared to dodge the hands.

Trivia

 * Despite a mid-development comment by Cenx that this would not be the case, the Moon Lord resembles common depictions of Cthulhu (it also fires "True Eyes of Cthulhu"), a creature first imagined by H. P. Lovecraft in his story, The Call of Cthulhu. This is partially a nod to broad fan speculation prior to the release of 1.3 that Cthulhu would be the final Terraria boss. This speculation arose in part due to an early concept design floated by the development team.
 * Terraria's final boss was originally going to be the Moon itself descending on the world, but Cenx later stated that this idea had been abandoned in favor of a new, unknown concept proposed by Crowno. The new concept was presumably the Lunar Events and the Moon Lord. The old concept may have been inspired by The Legend of Zelda: Majora's Mask, Final Fantasy VII, or the film Armageddon.
 * The Moon Lord bears similarities to a Tenome, a man-like creature with its eyes not in its face, but in the palms of its hands.
 * With a combined health of 145,000, the Moon Lord has the most health of any boss in the game.
 * The Moon Lord is the only enemy in Terraria that goes behind background walls. It is also the only enemy with a special animation when it spawns and dies.
 * When the Moon Lord is present, both the foreground and the background become very dark. Despite this, the Moon Lord is still fully visible.
 * After destroying the fourth pillar, but before the Moon Lord spawns, whatever music is playing starts to fade out, stopping completely a few seconds before he spawns.

History
Seigneur de la Lune