Thanos

"You will have your war, Asgardian. If you fail, if the Tesseract is kept from us, there will be no realm, no barren moon, no crevice where he can't find you. You think you know pain? He will make you long for something as sweet as pain."

- The Other describing Thanos' wrath

Thanos is a powerful alien warlord who rules over a dead dimension and commands a massive army known as the Chitauri. He allied himself with Loki to invade Earth.

Quest for the Infinity Stones
Thanos' past is unknown at this point and how he came to power. Not even his species has been fully discovered, sans the assumption that he is an Eternal. It is known that at some point, he gained control over the deadly army of the Chitauri, and took the assassin Gamora as his daughter. Thanos greatly desired the Infinity Stones, objects of immense power that held considerable dominance over the various forces that occupied the universe, and forged several alliances in order to locate them.

Helping Loki
"You question us? You question HIM? He, who put the scepter in your hand, who gave you ancient knowledge and new purpose when you were cast out, defeated?"

- The Other to Loki

At some point during his quest, Thanos discovered that the Tesseract, the Space Stone, was on Earth, and allied himself with the exiled Loki, providing him with the Chitauri, who would help him conquer the planet in exchange for the Tesseract. Once Loki had arrived on Earth, he succeeded in acquiring the Tesseract, but as he was preparing for his invasion, Thanos contacted by him. Through his intermediary, the Other, Thanos threatened the Asgardian god of mischief, saying that if he failed in delivering the Tesseract, Thanos would force him to endure a new kind of suffering.

After Loki was defeated, the Other reported to his master that the Earth's inhabitants were not as weak as they were thought to be. He also declared that challenging the Earth's heroes would be to court Death. To this, Thanos simply smiled.

The Collector
"One down, five to go."

- The Collector after recieving the Aether

When Thor defeated Malekith during the Convergence, he recovered the Aether. The Aether, which was actually one of the Infinity Stones that Thanos was seeking, was placed in the care of Taneleer Tivan, who, unknown to anyone at the time, was secretly in league with Thanos.

The Orb
Sometime later, Thanos' ally, Ronan of the Kree, recovered one of the stones and attempted to deliver it to Thanos, but an unexpected turn of events ensued and the stone fell into the possession of human pilot Peter Quill and his allies. Unwilling to be denied another stone, Thanos ordered Ronan to pursue Quill and recover the stone, sending his subordinate Nebula to aide the Kree in his pursuit.

Powers and Abilities
Thanos' powers are yet unknown at this point. However, it is conceivable that he is extremely powerful, able to strike fear into an Asgardian like Loki and command the Chitauri army unchallenged.

Relationships

 * Loki Laufeyson - Ally
 * The Other - Subordinate
 * Chitauri - Subordinates
 * Leviathans - Subordinates
 * Avengers - Enemies
 * Captain America
 * Iron Man
 * The Hulk
 * Thor Odinson
 * Black Widow
 * Hawkeye
 * Guardians of the Galaxy - Enemies
 * Star-Lord
 * Drax the Destroyer
 * Rocket Raccoon
 * Groot
 * Gamora - Adopted daughter and enemy
 * Nebula - Granddaughter
 * Ronan the Accuser - Subordinate
 * Korath the Pursuer - Subordinate
 * The Collector - Enemy turned ally

Behind the Scenes

 * Thanos' presence in the movie was kept top secret until the release of The Avengers.
 * Damion Poitier was credited as "Man 1" despite actually playing Thanos.
 * Kevin Feige has said that Thanos will likely be played by a different actor in future films.
 * Thanos is the only villain who has interacted, albeit indirectly, with every major hero in the franchise, as he was responsible for the events depicted in The Avengers (which featured all the major heroes of Phase One) and is the mastermind behind Guardians of the Galaxy.

Trivia

 * In the comics, Thanos has no relationships with Loki Laufeyson nor the Chitauri, but he's nevertheless one of the Avengers' most historical foes.
 * In the comics, Thanos is the master of the Infinity Gauntlet, which he used in the eponymous series in an attempt to court Death.
 * The name "Thanos" seems to be based on the Classical Greek θάνατος thánatos, meaning "death".