Born to A'Lars and Sui San of Titan, Thanos was born an Eternal with Deviant genes, a sight so horrible his own mother tried to kill him. Despite this, Thanos fit in well on his planet and wanted to be a scientist like his father.
Thanos even had a mysterious girl who would convince him to explore a cave with his friends. However, when a cave in separated him from his friends for 3 days, Thanos wouldn't kill any of the creatures for food only to discover that his friends had been murdered by the very same creatures he had let free. Later, Thanos would return to the cave and slaughter all the animals under the suggestion of the mysterious girl.
As a teenager, Thanos devoted himself to his studies and began to vivisect creatures for a reason as to why he was born with Deviant genes. The mysterious girl would become his confidante and she enabled him, helping him realise that he liked killing, but she always rejected his advances.
Thanos would leave Titan, joining a pirate crew and having multiple relations across the cosmos. However, he only felt something when he murdered his captain and thus, he returned to Titan, where the mysterious girl stayed. Enhancing himself, Thanos devoted himself to this mysterious girl who revealed herself to be the physical manifestation of Death.
This drove Thanos mad and he gave himself unto her, killing everyone on Titan with his pirate crew - sparing only his father and brother (Eros) - as he began his quest for Death.
Over the years, Thanos has searched for cosmic artifacts known as the Infinity Stones, which he used to wipe out half the universe. However, he would be foiled by nearly all the major players of the Marvel universe.
Thanos would fight Earth's heroes many times, until he was one day summoned to the future by an older version of himself called King Thanos. Without Death, his future self was worthless, and thus, Thanos returned to the present to find meaning.
POWERS: Thanos has incredible physical power, mental acuity, can manipulate energy, has cosmic awareness and has a cult known as the Black Order. Thanos is able to control cosmic artifacts like the Infinity Gauntlet.
WEAKNESSES: Subconscious need to lose.