Thanos is a character created by Marvel Comics in Iron Man #55 (February, 1973), written by Jim Starlin and Mike Friedrich, and penciled by Jim Starlin. It became a major character and antagonist, featured in many mini-series and events. He is one of the most popular villains in Marvel history.
Thanos is an Eternal, a member of an extraterrestrial superhuman subspecies 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.
As the Avatar of Death, he opposed the Many-Angled Ones (another name for the Great Old Ones) ruling over the Cancerverse and wishing to expand into Earth-616, invaded the prime reality, intending to kill Thanos to destroy Death, and fulfill their promise, "even death may die".
In the Cancerverse, his own counterterpart had been defeated and turned by the Many-Angled Ones.