His first act as the self-appointed champion of Death was to mercilessly kill millions of his own race via conducting a nuclear bombardment of his own planet, his first but by no means last act of mass-murder that earned him the fitting title of the Mad Titan. Thanos is a vast, complex and above-all-else powerful character thus trying to list every villainous act he has committed over the decades is no easy task - his most notable acts of villainy however have been the Cosmic Cube incident and the Infinity Gems saga - two times in which Thanos became, for lack of a better word, a God.
In the first incident Thanos, via a complex plan spanning multiple issues of many different Marvel titles, managed to obtain a cosmic artifact of immense power known as the Cosmic Cube which would grant whoever held it their greatest wish.
Thanos used the Cube to wish for omnipotence and easily defeated many of Earth's heroes but foolishly tossed away what he thought was now useless Cube - allowing the hero Captain Marvel to shatter the Cube and undo all of Thanos' evil - as a final blow to Thanos he discovers his beloved Death has abandoned him due to his failure, which while making him immortal by default is also deeply hurtful to the Titan - who is still in love with the entity.
Sometime later Thanos manages to gather the most powerful artifacts in the Marvel universe together (with some help from unwitting heroes and villains) - these Infinity Gems ultimately granted Thanos power so unbelievably vast he was able to erase half of all life in the universe without much effort and was all-but invincible even to cosmic-beings.
However Thanos, despite his omnipotence, would be defeated again when Adam Warlock managed to take the Infinity Gems away from the Mad Titan and undid all that the villain had done - Thanos, knowing he was defeated, faked his own death in order to escape. Thanos vs the AvengersAdded by SnakewhipPlease note these two events are by no means the only feats of evil achieved by Thanos but are merely some of his more memorable ones - Thanos is responsible for a great many other deeds of cosmic chaos and is almost always scheming up new ways to impress Death or gain unheard of power for himself.
Eventually Thanos was killed by his nemesis, Drax the Destroyer, who tore his heart out. However, in The Thanos Imperative, Adam Warlock encased his body in a cocoon through which he was revived, knowing he was the only means by which the Cancerverse, a mirror universe which was invading the Marvel universe, could be destroyed, since there was no death in the Cancerverse due to its leader,Lord Mar-Vell, sacrificing Death's avatar and killing Death of that universe.
Thanos was imprisoned by the Guardians of the Galaxy, until they realized that only he could kill the villains in the Cancerverse. Thanos was willing to go along with this to impress Death. Drax, overloaded by the life in the Cancerverse (being an Avatar of Life) destroyed Thanos, but he came back, as Death rejected him. Thanos killed Drax, and eventually confronted Lord Mar-Vell, but decided to let him sacrifice him for his ritual (to make the Marvel universe like the Cancerverse, as Thanos was its Avatar of Death), so that he could be reunited with Death, as he could not be killed any other way. However, Death arrived at the ritual and killed Mar-Vell, causing the Cancerverse to inevitable collapse. Death rejected Thanos despite what he did for her, and he became enraged, decicing to destroy the entire universe. Star Lord and Nova contained him inside the Cancerverse until it was destroyed, presumably killing all three. Thanos is immensely powerful, even when not empowered by outside magic or cosmic-devices, he is superhumanly strong and durable - he can manipulate and absorb cosmic-energy as well as alter matter in various ways and has displayed such varied abilities as telekinesis and limited telepathy. He is also in possession of vast alien technologies capable of time-travel, teleportation, force-field creation and entering alternate-universes. In addition to all this Thanos is knowledgeable, mind forever plotting new ways to get what he wants. It was speculated for awhile that Thanos might appear as a villain in The Avengers movie or one of its sequels. This rumor came from the fact that the Infinity Gauntlet appeared in the movie Thor in Odin's Treasure Room, and was stated to be important in later movies. Thanos' brief appearance during a mid-credits scene in The Avengers.Added by Rangerkid51It was later confirmed in the mid-credits scene of The Avengersthat Thanos was the one who provided Loki with his army of alien Chitauri as well as his staff (which contained the Mind Gem), getting in return the Tesseract (aka Cosmic Cube). Although he is not directly? seen until this scene, he is referenced a few times before this (though not by name). He is the one that the Chitauri's leader, The Other, is talking to at the beginning of the film, promising that he will gain control over the universe once he had the Tesseract. The Other warned Loki that if he failed to take over the world and give Thanos the Tesseract, there would be no place in the universe where Thanos could not find him, and would torture him so badly that he would "long for something as sweet as pain". When Thanos was informed that Loki failed because the humans had fought back, he decided to take matters into his own hands, as facing the humans would be to "Court Death".
Thanos appears as the post-credits villain in The Avengers 2: Age of Ultron, which released in aprox. 2015, were he gets the Infinity Gauntlet and proclaims he'll hunt for the Infinity Stones himself, although he first officially appears in the 2014 Guardians of the Galaxy movie bossing Ronin the Accuser around. It does not seem that he will appear in Thor: The Dark World, or Captain America: The Winter Soldier, which will feature Malekith the Accursed and Crossbones, respectively, as antagonists; however, it is possible that he will appear in post credits scenes. Thanos is the main penultimate antagonist of the 2018 film, Avengers: Infinity War and it's upcoming sequel.