Thanos is the main antagonist of the Infinity Saga and one of the overarching antagonists of the Multiverse Saga in the Marvel Cinematic Universe.
He was a genocidal Titan warlord who sought out the Infinity Stones in order to wipe out half of all life in the universe to save it from overpopulation. Commanding vast armies such as the Chitauri, the Sakaarans and the Outriders, as well as a devoted group of lieutenants known as the Black Order whom he adopted as his children, Thanos became one of the biggest threats in the entire universe. He is the adoptive brother of the EternalStarfox, and the archenemy of the Avengers and the Guardians of the Galaxy, particularly Iron Man, Thor, Gamora and Nebula.
Soon enough Thanos formed alliances with Loki and Ronan the Accuser to further ensure the accomplishment of his agenda, but those partnerships failed as he was betrayed by both of his supposed allies. Thanos' actions inadvertently led to the creation of Ultron, as his supply of the Mind Stone to Loki granted the rogue AI its own free will. Undeterred, he later obtained all six Infinity Stones and successfully wiped out half the life in the universe.
His victory was short-lived after he was executed by Thor, and five years later when the Avengers successfully retrieved the Infinity Stones through time travel. Their attempt to undo his actions, however, ends up attracting the attention of a past version of the Mad Titan, who traveled to the present with stolen Pym Particles in an effort to annihilate the Avengers and erase then recreate the universe according to his ideal vision. In the end, Thanos was ultimately defeated upon being erased from existence himself alongside his army.
In The Avengers, he was portrayed by Damion Poitier. For the rest of the MCU, he was portrayed and voiced by Josh Brolin through motion-capture.