Thanos

Thanos's first appearance lasts only a few seconds. In the mid-credits scene of The Avengers, the Other, his servant, warns that attacking Earth would be courting death. Thanos turns and smiles. It becomes clear he's the one who gave Loki the Chitauri army and the scepter in exchange for the Tesseract. In that scene the character is played by Damion Poitier. Josh Brolin takes over the role starting with Guardians of the Galaxy, through motion capture.

In Guardians of the Galaxy, Thanos appears seated on a floating throne, sending Ronan the Accuser to fetch an orb hidden on Morag. Gamora and Nebula serve Ronan on his orders, and both are Thanos's adopted daughters.

When Ronan finds out the orb holds an Infinity Stone, he decides to keep it and threatens Thanos himself. Gamora had already betrayed her father to escape that life.

In Guardians of the Galaxy Vol. 2, Nebula describes their childhood: Thanos made them fight, and every time Nebula lost, he replaced a part of her body with a mechanical piece.

In the mid-credits scene of Avengers: Age of Ultron, after two failures by his underlings, Thanos puts on a golden gauntlet and decides to go after the stones himself.

Avengers: Infinity War is Thanos's movie. He starts with the Power Stone, already taken from Xandar, and attacks the Asgardian ship: he kills Heimdall and Loki and takes the Space Stone. His adopted children, the Black Order, go to Earth after the Time and Mind Stones.

The movie explains his motive. Titan, his home planet, collapsed from overpopulation and lack of resources. Thanos proposed killing half the people at random and was called mad. After that, he started carrying out the plan on other worlds. That's how he found Gamora as a child, on her planet, and took her with him.

On Knowhere, he takes the Reality Stone and captures Gamora. Seeing Nebula being tortured, Gamora reveals where the Soul Stone is. On Vormir, its keeper, the Red Skull, explains the price: losing someone you love. Thanos weeps and throws Gamora off the cliff.

On Titan, he fights Tony Stark, Doctor Strange, Spider-Man and the Guardians, and almost loses the gauntlet. Strange hands over the Time Stone in exchange for Tony's life. In Wakanda, Wanda destroys the Mind Stone, but Thanos turns back time and rips it out of Vision's head. Thor buries his axe in Thanos's chest, missing the head. Thanos snaps his fingers, and half of all life in the universe turns to dust. In the last scene, he rests in a hut and watches the sun rise.

In Avengers: Endgame, the surviving Avengers find Thanos 23 days later, on an isolated planet where he lives as a farmer. The stones no longer exist: he used their power to destroy them. Thor cuts off his head.

The threat returns through time travel. A Thanos from 2014 learns of the heroes' plan through Nebula's memories, crosses through time with his army and bombs the Avengers compound. He beats Thor, Tony and Cap, breaks the shield and says he'll now destroy the entire universe to rebuild it. In the final battle, Tony takes the stones from his gauntlet, snaps his fingers, and Thanos crumbles to dust along with his army.

In the mid-credits scene of Eternals, Eros appears, introduced as a prince of Titan and Thanos's brother, played by Harry Styles. It's the only hint about the villain's family on screen so far.

Thanos was created by Jim Starlin and debuted in The Invincible Iron Man #55, in 1973. In the comics, his motive isn't scarce resources: he's in love with Death personified and kills half the universe to impress her, in the 1991 saga The Infinity Gauntlet.