A warlord from a dead world who believes the universe has too many people in it, and who spends decades assembling the means to remove half of them.
Thanos was born on Titan roughly a thousand years ago, watched his world use itself up, and proposed killing half the population at random to save the other half. Titan refused, exiled him, and then died exactly the way he said it would. Everything after that is a man who was right once, generalising.
The films are unusually careful with him. He is not given a lie to hide behind. He believes what he says, he applies it to himself, he takes no pleasure in the arithmetic, and the story does not have anyone deliver a speech proving him wrong. It simply shows the result, at length, across two films, and lets the grief make the argument.
He also owns the structural trick of the whole Infinity Saga. He is set up for six years in scenes lasting seconds, in films that have nothing to do with him, until the promise of him is large enough that the film with his name in it barely needs an introduction.
Show detailsA villain assembled in fragments
Before Avengers: Infinity War he has almost no screen time. A turn to camera after the credits of The Avengers, a throne and a conversation in Guardians of the Galaxy, adopted daughters who talk about him more than he appears. He is the subject of other people’s fear for years before he is a character.
The Wikipedia article on him says he appeared in five films of the Infinity Saga without naming the fifth, and this catalogue lists four, because those are the four the article documents. If you find the fifth in a source we can check, the entry is easy to correct.
Major spoiler
Thor kills him. Early in Avengers: Endgame the surviving Avengers find him on the planet he retired to, learn that he has destroyed the Stones so that nobody can undo what he did, and Thor takes his head off with Stormbreaker. It changes nothing, which is the point of putting it at the start.
The Thanos of the last act is an alternate version from 2014, who finds out about the Blip through the cybernetic link between the Nebula of his time and the Nebula of 2023, and who decides he would rather destroy the universe and build a new one. Tony Stark takes the Stones off him and ends him and his entire army.