Majora

"According to legend... the troubles caused by Majora's Mask were so great... the ancient ones, fearing such catastrophe, sealed the mask in shadow forever, preventing its misuse. But now, that tribe from the legend has vanished, so no one really knows the true nature of the mask's power... ...But I feel it."

- Happy Mask Salesman

"A puppet that can no longer be used is mere garbage. This puppet's role is finally over..."

- Majora

Majora is the main antagonist of The Legend of Zelda: Majora's Mask. It is a demonic being.

Biography
This biography of Majora is organized into the latest "official Zelda timeline", this may change completely in the future as the timeline given is merely meant as an example, and their have been official timelines before it.

Child Timeline
This is the timeline where Link was sent back to the past and was able to warn the Royal Family of Ganondorf'f treachery, leading to the events of Twilight Princess' backstory.

Majora's Mask
Majora inhabits a cursed mask that was used by an ancient tribe in it's rituals. It's intent is to destroy the town of Termina by bringing down the Moon on top of the city. For the majority of the game, the mask is worn by the Skull Kid on the Clock Tower, drawing the Moon to Termina. Although the Skull Kid was unaware, Majora's Mask gradually began to take complete control of his mind, turning his harmless pranks on people to acts with malicious intents. Before the game's events, Skull Kid used the power of the mask to prank and curse others around him, such as Kafei.

Decline Timeline
This is the timeline where Ganondorf killed Link in his final battle with him, leading to the Imprisoning War described in A Link to the Past's backstory.

A Link Between Worlds
Majora's Mask (or it's Lorule counterpart) appears as one of the items in Ravio's shop. It is also mentioned that the game is linked to Majora.

Super Smash Bros. Melee
Majora's Mask appears in a stage (only playable in Event 47). If the player defeats three random opponents without losing both lives, they unlock Majora's Mask trophy.

Super Smash Bros. 4
Majora's Mask, along with Skull Kid, have been confirmed to appears as an assist trophy in the next Super Smash Bros. game.

Hyrule: Total War
Majora appears again in the Hyrule: Total War by Undying Nephalim, in this version he is in his true form and the leader of the Church of Majora. This game states that the goddess sealed him away a long time ago. He is the final boss of the game, though only his followers are fought in the final level.

Trivia

 * Majora is possibly the only villain in the franchise with unclear motives. Rather than try to take over the land or gain amazing power, he seems to only wish to spread as much chaos and misery as possible.