Thanos is a villain in the Marvel Universe. He possessed the Infinity Gauntlet for a short time, becoming a threat to reality itself.
Thanos was born on Titan, one of Saturn's moons. Due to a genetic quirk, he looked very different from his fellow people. He was treated fairly, but remained very distant, only speaking to his brother. He studied bionics and mysticism and became very powerful. He was often referred to as "The Mad Titan." He became nihilistic and fascinated by death. He bombarded Titan with nuclear weapons, killing millions of his own people. Years later, Death manifested itself to him in the form of a woman, whom Thanos fell in love with. Determined to win her approval, he set out to find a way to destroy all life in the universe. His search eventually led him to the Infinity Gems, each of which he gathered by besting an Elder Of The Universe. He then fashioned the Infinity Gauntlet. Unfortunately, possessing all of the Infinity Gems put Thanos above Death, and she could not acknowledge him. Determined to win her approval, he erased half of all life in the universe from existence. He went on to defeat Earth's heroes, and then a host of cosmic entities, including the lords of chaos and order, Galactus, and finally Eternity, the embodiment of the universe itself. He then ascended to become the new Eternity, losing his corporeal form, and thus his gauntlet. This allowed Nebula, Thanos' supposed granddaughter, to gain control of the gauntlet and undo everything Thanos wrought. Defeated, Thanos activateed a nuclear device, preferring death to imprisonment. Unable to disable the advice, Thor used his mystical hammer to transport Thanos away from the heroes, and Thanos teleported, leaving his device behind to explode, faking his death. He reformed for a time, but has since gone back to his villainous ways.
Marvel Cinematic Universe
Thanos has appeared in the Marvel Cinematic Universe, starting with a brief cameo in The Avengers (2012), played by Damion Poitier. He was acted by Josh Brolin in Guardians of the Galaxy (2014) and Avengers: Age of Ultron (2015) before Avengers: Infinity War (2018) and Avengers: Endgame (2019).