Born to the Eternals Mentor and Sui-San, Thanos was subject to the Deviant gene at birth. As a result, Thanos became like that of a 'mutant' of Titan (the sixth orbiting moon of Saturn) where he developed a purple, hide-like skin along with strength and other physical abilities far surpassing that of other Titans.
This mutation also augmented the powers he inherently possessed as a descendant of the Eternals. Growing to become the most powerful Titan, his skin developed in such a fashion so as to allow the Titan to not only absorb cosmic energy on a atomic level, but also then manipulate it as kinetic force via conscious choice.
Though never shunned by his family as a youth, Thanos was feared and shunned by his fellow Titan peers as a result of his unique qualities. His brother, Eros, was groomed from their childhood to eventually replace their father in the role of what constituted a leader among their people. Eros would, throughout their youth, be fawned over by loving friends and family, whom much favored him over Thanos. Feeling the sting of such ostracism, Thanos would find solace and purpose in other pursuits. On one fateful day, in an all but forgotten subterranean temple, an enterprising young Thanos would find companionship in the form of Mistress Death.
Decades would pass and Thanos' relationship with Mistress Death grew and eventually blossomed into a dark and forbidden romance. Under Death's tutelage, he grew even stronger and more powerful - well versed in the Black Arts, which had long been forbidden on Titan. She taught Thanos that knowledge is power, and that power is everything.
With the revelations of Thanos' rebellious activities, his maturation, and rebellion against his father: The Mentor, Thanos' ensuing exile was not far from fruition. When his father discovered Thanos conducting forbidden dark art experiments, he expelled Thanos from Titan - though not without a heavy heart. Fueled by his hatred for this slight, Thanos would begin to amass even more knowledge, skill and power via mysticism, meditation, and bionics during his century long isolation.
Eventually, having surpassed all other Eternals in power and strength, Thanos decided to return to Titan to display his new found might. Thanos would not come home with the intentions his Father had hoped for after his punishment. Instead of atonement, Thanos attacked the planet with nuclear weaponry while orbiting Titan. Thanos had miscalculated Titan's defenses in his haste to sate his anger, which allowed some to escape the onslaught. However, only a handful of what was once thousands, survived the invasion. As chance would have it, neither Thanos' brother nor father were on Titan during the assault. To Thanos' regret though, his mother was not so fortunate - having stayed on Titan, as she was the one member of his family he might have spared. In the aftermath, Thanos left the remains of Titan and returned to space. Proving his devotion and worth to Death was all that mattered to the Titan. with his home now crushed in his wake. During this time in the cosmos Thanos conquered and pillaged many worlds acquiring vast resources such as an assortment of followers, technology, weapons and space craft.
At this point, Kronos, the father of Mentor (Alars) and the earth based Eternals All Father Zuras, sought to act out of a sense of obligation for what Thanos had wrought and might yet further cause. Mentor pleaded with this cause to Kronos that they might prevent future tragedy. To that end, Kronos created Drax the Destroyer to ward away the ill which Kronos knew to be the future for The Mad Titan. Drax was formed as an avator for the Eternal - built out of the very cosmic power which made up the living Eternal. Into Drax, Kronos would imbue the singular mission of seeking out Thanos to stop his designs. Drax would become the cosmic force to act as a foil to Thanos. Setting out upon his task, Drax would meet The Mad Titan on a living planet distant from our own star system. The two mighty beings met in an incredibly violent physical confrontation. The clash rendered the landscape asunder and the planet was destroyed - with the two combatants alone to drift within the debris. However the planetary explosion would render Drax incapacitated, allowing the still conscious Thanos to easily capture the Destroyer. After having dispatched Drax in such a fashion, Thanos set about further plans for stepping stones in his growing empire. Taking an interest in Earth - Thanos sent his henchmen the Blood Brothers to survey and deal with a potential obstacles to his ambitions - one of which would become Iron Man. At this time in the cosmic prison Thanos left him - Drax awoke to the coming danger of Thanos' plans. He then would reach out telepathically - warning Iron Man of the danger nearing his planet and of the history the Titan had for violence. Yet the message was too late as the brothers apprehended Tony before receiving the distress call of Drax. The brothers would return Tony to Thanos staging grounds. During this time Tony drew upon an escape plan and ambushed the Brothers in an unsuspecting moment. Though able to catch the Brothers off guard - Thanos became aware of the scuffle and stepped in. Thanos would crush the metal gloves around Tony's hands under his heel while Tony lay prone from a singular Titan blow. However - Mentor had been monitoring events from Titan and fired a unknown energy that reached the Titan - simultaneously saving Tony and setting Drax free. Thanos wasted no time deploying the brothers as a distraction while he set the self destruct sequence and made his escape. After a fierce battle against the armored hordes of Thanos - the heroes then attacked the Titan himself. Yet to the heroes' dismay they in fact attacked a robot look alike, left to keep them occupied until the ensuing explosion which would allow for the Titan's full retreat. Recognizing the danger, Mentor was able to ward the heroes away before the explosion reached critical mass. In the aftermath, Thanos' fortress was destroyed, but his whereabouts were unknown.
True to his beginnings,Thanos ever seeks out the mysteries to power for personal use or as a ward from enemies seeking retribution. Yet even the quest for power is dwarfed by endeavors or errant missions for Mistress Death in an attempt to curry her favor. Always Thanos seeks power - but is forever wed to his quest to be found worthy as Death's cohort. He has succeeded on multiple occasions, but has been consistently defeated. Most often though, as in the case of the Cosmic Cube, The Infinity Gauntlet and lastly in the story told in Marvel: The End - it is a result of a deep psychological construct. In critical moments during these events, Thanos provides a window to his enemies into his subconscious which belies the belief that he does not deserve or even want the power he seeks. This has been most often presented by the personification of his foil - Adam Warlock - whom maintains a prescient link to Thanos through their extended contact within the Soul Gem.