Thanos is a supervillain in the fictional Marvel universe, who was created in 1973 and has been one of it's main supervillains, an A-list supervillain with several mini-series. He is one of the most popular villains in Marvel history.
Thanos is an ancient Eternal, a member of an extra-terrestrial superhuman sub-species of mankind who is over a thousand years old. At a young age he became fascinated with death and encountered the cosmic abstract Death. Who seduced him and ever since he has been in love with Death and obsessed with her, willing to do anything for her. He left his homeworld, became a pirate, and then an intergalactic warlord who has destroyed worlds, slaughtered races, and collected tribute.
Eventually his ambitions came to encompass all of the universe, and has made multiple attempts. The closest he came was with the Infinity Gauntlet, becoming the most powerful being in the universe, higher than the cosmic pantheon of the cosmic abstracts. At times he has worked alongside with the superheroes of Earth and/or the cosmos, sometimes because he is bored and sometimes because someone poses a threat to reality. One of those times was when one of the realities which had been conquered by the Old Ones, the Cancerverse, invaded the prime reality and played a key role in its defeat.