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, or by using a Celestial Sigil. The Moon Lord spawns about one minute after either of the prerequisites are met. It serves as Terraria's "final boss" but theories suggest that Cthulhu might be the final boss.

If all players die during the Moon Lord's encounter, the Moon Lord will despawn. It can 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 boss again), or by using a Celestial Sigil.

In Expert Mode, he also drops a light-pet summoning item, the Suspicious Looking Tentacle, as well as the Gravity Globe, an accessory which allows the player to change gravity whenever they like. It basically serves the purpose of an infinitely lasting gravitational potion, which is useful in many situations.

Behavior
The Moon Lord has two hands and a head that must be killed before its "core" becomes available for attack. It positions its two hands on either side of the player some distance away.

Each hand, as well as its head, contain a vulnerable eye-like target 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.

The Moon Lord can teleport to players if all of them get too far out of range.

Attacks

 * The head target will periodically fire a "Phantasmal Deathray", the Moon Lord's most powerful attack.
 * All three Eyes fire "Phantasmal Bolt" high-speed laser bullets.
 * The hands spawn "Phantasmal Spheres", invincible eyes that charge at players.
 * The hands also fire "Phantasmal Eye" explosive homing projectiles at players.
 * Once a hand or head target is killed, a True Eye of Cthulhu spawns, which fires lasers or ice at the player.
 * Each hand target killed becomes a thorny crevice thereafter that deals damage on contact.
 * The mouth periodically attempts to attach a tentacle to the player, which spawns multiple Moon Leech Clots, essence-like monsters that travel from the player to the Moon Lord's mouth. If not killed by the time they reach the mouth, one of the Moon Lord's parts (one that has not yet been defeated) is healed for 1000 health for each of them.
 * The Tentacle also inflicts the Moon Bite debuff, preventing any life-steal effects from weapons/armor like Vampire Knives or Spectre Armor for 14 seconds (16 seconds in Expert mode).

Tips

 * The easiest way to defeat Moon Lord the first time is to make a house and assign a nurse to it, attacking Moon Lord from the inside with Solar Eruption. If you close the dialogue with the nurse after healing, she will heal herself rapidly and it is very rare for her to die.
 * Using Stardust Dragon staff with Tiki Armor or Stardust Armor (if you have already beaten Moon Lord) with nurse (for healing), you can defeat Moon Lord in under a minute.
 * 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. Another one, if you have beaten all the Mechanical Bosses in expert mode, is to make a huge Minecart track with bumper ends and use the Mechanical Cart. The bumper ends makes it easier to turn when the track ends, making it even harder to be reached by his tentacle.
 * 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.
 * 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.
 * It is possible to avoid the laser beam that the first-stage Moon Lord fires at you using the Cosmic Car Key mount by staying close to where the laser beam is fired from (as opposed to farther away from where the laser is fired), and flying over the Moon Lord's head. The Rod of Discord can alternatively be used to teleport behind the laser as it is firing, freeing up a lot of time while it is firing elsewhere to attack the head. It is also possible to fly above the beam with the Fishron wings.
 * The Cosmic Car Key, depending on which direction you are traveling at the same time as Moon Lord, may not gain enough velocity to outrun his own when he starts to rise. Rely upon Solar Wings, Fishron Wings, or a Minecart instead. An accessory that allows you to dash, such as the Master Ninja Gear, may also be useful for keeping a safer distance.
 * A large portion of the Moon Lord's attacks, including the laser he emits from his head, are incapable of passing through walls. Consider seeking cover to avoid these attacks during the fight.
 * Having the Nurse nearby to constantly restore your health greatly increases your chances of beating the Moon Lord or you could set your spawn to the house where the Nurse lives and teleporting using the Magic Mirror, Recall Potion etc. whenever your health gets low. The nurse is usually able to heal herself faster than the Moon Lord can damage her, however, moving the nurse out of the town can prevent any other NPC deaths.
 * It is not wise to use Vampire Knives or the Spectre Hood as the Moon Bite debuff occurs very often, which disables life steal from these weapons/items.
 * Consider using an actuator roof with a lever attached to it, as the Phantasmal Deathray cannot travel through blocks. Turn on the roof whenever the Moon Lord is using his Phantasmal Deathray.
 * Using automatic spiky ball traps and minions, you can damage Moon Lord from under the trap machine with both traps and minions. Keep moving under the machine in a mechanical cart to avoid any of the three eyes. Spooky armor may help in the process but taking damage at all is next to fatal. The traps should block all but the eye melee attack. Xeno Staff or Stardust Dragon Staff are good choices in minions.
 * At all costs, try to avoid his laser that he shoots at you. It can deal tons of damage, so while building your arena, put blocks above you for cover. If you don't, your chances against Moon Lord is serious and won't be easy, especially in expert mode.
 * Unlike every other boss, the Moon Lord does not despawn if you go too far away from him. Rather, he teleports to where you are. This can make the fight actually easier than others.
 * It is also possible to not get hit by some of the attacks he does, you simply build a small building with background walls to not be hit by the Phantasmal Deathray and melee from his hands, but does not stop the True Eyes of Cthulhu, this will make the fight so much easier if you have a weapon that can go through walls (e.g. Solar Eruption).
 * When the Moon Lord teleports, he always teleports to the player with the shortest distance from him.

Trivia

 * Despite a mid-development comment by Cenx that this would not be the case, the Moon Lord resembles common depictions of Cthulhu, a creature first imagined by H. P. Lovecraft in his story, The Call of Cthulhu. Fans commonly speculated prior to the release of 1.3 that Cthulhu would be the final Terraria boss (which in turn arose in part due to an early concept design floated by the development team). Cenx later stated that the Moon Lord actually represents Cthulhu's brother.
 * 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.

History
Лунный Повелитель Seigneur de la Lune 달의 군주 Moon Lord