Thanos is the ruler of the Endless Resurgence, a vast empire centered on the planet Acheron consisting of thousands of worlds that exists on another plane of existence.
Thirty-thousand years ago, a powerful artifact known as the Cosmic Cube mysteriously landed on Acheron. Thanos found the artifact and used it in his conquest of the universe and expressed his will and influence to extinguish the will of anyone who opposed him and his "order". His expansion with the Cosmic Cube was halted when the Cube was destroyed by Mother Induna of Halcyon with the cost of her life.
Though his expansion was halted, Thanos still rules his conquered empire with his draconian policies in which any resistance from his subject planets are brutally suppressed. In the present, Thanos captured Halcyon's most powerful Seed warrior, Tesseract the Everywhere Man, and attempted to brainwash him to serve his empire. However, when Seed Nineteen were able to rescue Tesseract and joined forces with the Fantastic Four, Thanos knew that Reed Richards would be able to construct the Cosmic Cube. Thanos first personally met Reed by possessing Ben Grimm and "requested" him to construct the cube with coercion by killing a weakened Johnny Storm. Fortunately Ben was able to resist Thanos' possession. Though Reed refuse to build the Cube for Thanos, he however, intends to build it to use it against Thanos.
Eventually the Fantastic Four came into a second conflict with Thanos in which he was able to harness Reed's Cosmic Cube and once again used it to express his will by conquering Earth and brainwashed Earth's heroes as his pawns to fight against his enemies. However, when Thanos attempts to utilize the Cube to ascend into godhood, he suddenly realized that there was something wrong and is instantly killed. His (hopefully) permanent death was elaborated that Reed had built a "cut-out" into the Cube so that it would only respond to conscious, willed thoughts, and that he removed this "cut-out" while Thanos was distracted and which allowed his death. With Thanos' death and the restoration of his past actions of utilizing the Cube, Reed disposed the Cosmic Cube in a time-rift creating a paradox in which the present events were the cause of Thanos getting the Cube in the past.
Thanos possessed immense physical strength (lifting over 100 tons) and endurance. His psychic powers allowed him to have awareness of events several star systems away, and he could take over other's bodies, using them as conduits to channel his mind and power until his mind exhausts and kills the frail host body. He could also rise from the dead, sacrificing other lives to resurrect his own, as well as recharging his energies to awaken more powerful the longer he stayed dead (he had remained dead a little over 49 days). He had also claimed he could transfer his follower's souls from one body to another, and control what form their next life would take after they died.
The Ultimate Thanos bears strong similarities to Jack Kirby's and DC Comics' New Gods villain, Darkseid. The similarities between the two villains is that they rule their domains with draconian policies, and their desire of a all powerful cosmic source (Thanos wants the Cosmic Cube, and Darkseid wants the Anti-Life Equation) to exert their rule over the universe.