Thanos was the absolute ruler of Titan, would-be conqueror of the universe and the greatest enemy of the Avengers. He psychically fed on the despair and misery of others. Thanos has conquered and subjugated an unspecified number of planets throughout the universe and his quest is to obtain Death and the Infinity Gauntlet.
Thanos was a Eternal peasant born with a deformity — the Deviant Genre, which reduced his purple skin and monster appearance. For that reason he was cast out, and went to wander in the wasteland. One day, when he was jumped by a group of criminals, Thanos was saved by the mythical Death, who decided to give him a chance. It infused him with great physical strength and mental powers, which he could project through his intelligence. Moreover, Death revealed itself to be sentient when it told Thanos of a time when Titan would become a paradise. Consumed by anger and hatred, Thanos misunderstood this message and believed that he had been chosen to impose his will upon the galaxy, be alongside Death as her love and as a result, he used his powers to subjugate his fellow natives.
Thanos quickly established himself as the ruler of Titan, his once-reviled deformity now acting as his link to Death's dimension. With an entire planet in his hands, he built an empire called the Chitauri. Thanos' selfish misuse of Death's words twisted it into a source of hatred, fueling his despotic rule. He killed all those not worthy of reading and punished anyone who opposed him. He was often accompanied by his loyal servants, most notably, Corvus Glaive.
Thanos then seek to bring the entire universe under his control. To achieve this end, he seeks the cosmic power of the Infinity Gauntlet, which theoretically will enable him to control any and all existing life in the universe. Thanos' perpetual rival is Adam Warlock, ruler of the planet of Xandar. For thousands of years, Titan and Xandar waged war, claiming millions of lives until an truce was finally agreed upon. To seal their bond, Warlock and Thanos exchanged their infant children: Adam would look after Thanos' daughter Gamora and Thanos would raise Adam's son Peter Quill. Peter, despite the influence of Thanos and his daughter, Gamora, grew up to become a freedom fighter in opposition to his adopted father, adopting the name of Star-Lord. Gamora took to the lessons of Xandar and became an enemy to Thanos as well.
Though maintaining an uneasy peace, Thanos still looked to acquire power without Adam's interference. He soon took notice of the planet Earth, not only for its vast natural resources and vulnerable population, but also because a potential rival had appeared amongst the otherwise "unworthy" masses: the Avengers. In an effort to gauge the Earth's Mightiest Heroes power (as well as prepare Earth for his coming) Thanos aided Loki by supplying powerful weapons and the Mind Gem of his world to the greedy Asgardian god of mischief.
Thanos ordered his servant Corvus Glaive to destroy the Avengers, but his device failed. After his daughter Gamora disobeyed orders by battling The Avengers, Thanos made his presence known to the Earth's heroes, injuring Iron Man with his laser eyes before disappearing into a Chitauri portal.
Thanos would later launch a full attack on Earth, beginning with the massive destruction of a nuclear facility. The Avengers were again effortlessly defeated in personal combat and bound to the head of Thanos' war machine. The invasion would have succeeded if not for the timely intervention of Adam Warlock and the armies of Xandar. With Warlock declaring that Earth was off-limits, Thanos grudgingly retreated, but not before murdering Happy Hogan in front of all of New York, then escaping through a Chitauri Portal before Iron Man could get to him.
In the quest for the Infinity Gauntlet, Thanos invaded a sector protected by Xandar, thus violating the rivaling planets' ancient peace treaty. Gamora's fleet was waiting for Titan's invasion force and shattered Thanos' armada. Upon this, Adan Warlock gave Thanos an ultimatum assuring that the next breech of their treaty would result in his annihilation. While Titan's was rebuilding it's military forces, the villainous Earth android Ultron arrived with the intent of assimilating the planet. Thanos approached Ultron and offered him to hand over Ant-Man and in return, Ultron would spare Titan. So, Thanos went falsely to the Avengers and asked for their help to battle Ultron, which the team reluctantly conceded. In Titan, Thanos and Iron Man fought together until Ultron's forces retreated. Thanos told the Avengers to follow and finish him off, which they did.
Afterwards, Thanos teleported inside Ultron's headquarters just in time to vanquish the Avengers, thus delivering his end of the bargain: Ant-Man. When Ultron started extracting living DNA from Pym, Thanos implanted a device that overrode Ultron's control circuits, turning him into his puppet. Thanos intended to use Ultron's program to collect the Infinity Gauntlet and reorder the universe at his will. At that point, Gamora, Iron Man, as well as Captain America showed up to thwart him. Gamora and Thanos exchanged a couple of punches, but Thanos was victorious. When Thanos attempted to leave Ultron's station, however, he was intercepted by Iron Man, who was determined to hold him inside until its destruction. Thanos fought with everything he had, but was finally stranded within the exploding headquarters, dying in the blast. Despite the circumstances, Thanos rejoiced and scorned the Avengers in his last breath.