Thanos (Marvel Cinematic Universe)

'''"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.

Role
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 Garmora as his daughter. Thanos greatly desired the Infinity Gems, 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.

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 heroes would be to court Death. To this, Thanos simply smiled...

Thor: The Dark World
When Thor defeated Melekith, he recovered the Aether. The Aether, which was actually one of the Infinity Stones that Thanos was seeking, was placed in the care of Teneleer Titan, who, unknown to anyone at the time, was secretly in league with Thanos.