Moon Lord

The Moon Lord is a boss that will be included in the Terraria 1.3 update. It is presumably battled after all four Lunar Events have been completed, serving as a "final boss" to the game.

Trivia

 * The boss was originally planned to be the moon itself descending on the world, however Cenx later stated this plan had been abandoned in favor of a different concept, presumably the Lunar Events.
 * This old concept could possibly be a reference to The Legend of Zelda: Majora's Mask.