Thanos

Overview
Thanos is a large, muscular purple-hued alien from the planet Titan. He had a daughter named Nebula, and an adoptive daughter named Gamora. Thanos came to believe that the universe did not maintain enough natural resources to sustain itself, and that entire civilizations were being driven to the point of starvation due to overabundant populations. To resolve this crisis, Thanos believed that a mass culling was in order. Along with his army, which included the Black Order - the so-called "Children of Thanos", as well as hordes of alien Chitauri, he went from world to world decimating half of a planet's population. On one such world, Thanos took in a newly-made orphan named Gamora, whom he took under his wing as his daughter.

Thanos believed that this genocidal culling was his destiny, but his current methods were not producing the desired results quickly enough. To speed up the process, Thanos began seeking out the Infinity Stones - six ingots that have existed since the dawn of creation that, when combined, would yield him ultimate power. Thanos also employed other agents to procure the stones, such as Loki Laufeyson, whom he sent to Earth to find the Tesseract, and Ronan the Accuser, whom he sent to Knowhere to find the Power Stone.

Thanos did not get the Tesseract until the Black Order and he boarded The Statesman - a space vessel carrying Asgardian refugees. He claimed the Tesseract after killing Loki. He then decimated the planet Xanar, which was now keeping hold of the Power Stone. After which, he went to Knowhere, where the Collector had the Aethyr - the Space Stone. During this time, he had a confrontation with Gamora and her new allies, the Guardians of the Galaxy, whom he easily defeated.

The remaining three stones were the Soul Stone, the Mind Stone, and the Time Stone - two of which were on Earth. He then traveled to the planet Vormir, where the displaced Earth Nazi Johann Shmidt, aka the Red Skull, was the caretaker of the stone. Acquiring the stone required sacrificing something dear, so Thanos killed Gamora in order to get the stone.

He then went to his homeworld of Titan where he fought up against the Guardians, as well as Spider-Man, Iron Man, and Doctor Strange. Strange knew that there was only one possible future outcome in which the Avengers would be victorious, and that required giving up the Time Stone, which Strange kept in an amulet on his person.

With only one stone left, Thanos traveled to Earth. The Mind Stone had been worn on the brow of an android hero known as the Vision. The Vision's lover, Wanda Maximoff tried to defend the robot, but Thanos destroyed him, and took the stone. The Asgardian God of Thunder, Thor Odinson, having just arrived on Earth after procuring a "Thanos-killing weapon" called Stormbreaker, engaged Thanos in direct combat. He cleaved the weapon into Thanos' chest, but by now it was too late. Thanos had already acquired all six Infinity Gems. By snapping his fingers, he instantly caused one half of the entire galaxy to disappear - effectively killing untold billions of people.

Feeling as if his task had been completed, and his destiny fulfilled, a tired and injured Thanos retired to a faraway world to live out the rest of his days.