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 an evil being, he was responsible for possessing Skull Kid and trying to destroy Termina. He was obliterated by Oni-Link, the Fierce Deity (Fierce God) and was sealed into his inanimate normal mask and was given by The Happy Mask Salesman for good.

History
This history 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. However, this serves as nothing more than decoration, and possibly a hint about Majora's Mask 3D being in development.

Powers and Abilities
The full scope of Majora's powers are unknown. But, they are magical in nature with dark themes. Examples, are when the Skull Kid used the mask, to seal the four giants Termina's (gods) into mask with each temple. He also used the mask's magic to curse Link into a deku and turned Kafei into a child. It even has the power to affect the afterlife, as it cursed one area of Termina with Poes and Undead.

The temples were also cursed and affected the land near each one. The Southern Swamp's water became poisoned. The Snowhead Mountain had a perpetual winter. The Great Bay's ocean had murky water. Ikana Canyon was cursed with Poes and undead. From different kinds of curses, to possessing a host as well the Mask that contains it's power and spirit. It's power is also great enough to pull the moon, into Termina and was going to crash into Clock Town. After Link defeated Majora itself, the mask seemly lost it's evil and dark magic.

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 appear as an assist trophy in the next Super Smash Bros. game. When Skull Kid is summoned, he works similarly like Tingle, but more sadistically. His abilities include:
 * Flipping the stage upside-down.
 * Turning all the fighters invisible.

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.