In the as kill nearly every single hero in the Marvel Universe roster. As shocking and impressive as that feat proved, however, the Mad Titan proved to be an even more terrifying, all-consuming force some time AFTER he'd acquired (and lost) the Infinity Gauntlet.
Thanos: The Infinity Conflict is the 2018 Jim Starlin-conceived story in which Thanos's most terrifying form appears. Beginning with a flashback in which the cosmic hero Adam Warlock hands off the Reality Gem to Thanos for safe keeping (his logic being that no one would dare steal a cosmic stone from the Mad Titan), the story jumps ahead several years, revealing that Thanos has been directed by his future self into securing several cosmic relics. These relics --- in particular a special "regulator" that in part keeps the fabric of this reality, which itself is hidden deep within a comet --- in part imbue the Mad Titan with immense cosmic powers, with which he is able to overpower and absorb some of the most powerful celestial beings in existence.
As Adam Warlock, accompanied by Pip the Troll, trots around the cosmos trying to uncover Thanos's plan, he is continually killed, only to be resurrected some time and distance from his point of death. With Adam Warlock mostly out of commission, it comes down to Eros, Pip, and Eros's supercomputer I.S.A.A.C. to track Thanos through time, space, and reality itself --- particularly when it is revealed that the massive spaceship of Galactus, crashed and broken. Emerging from within, massive and on a scale rivaling that of the Devourer of Worlds himself, is Thanos.
The revelation of Thanos's newest form is an awesome tale of celestial might. Having been spurned by his love, Mistress Death, Thanos has transcended the rules of reality itself --- in fact, only three such beings have and can transcend death: Thanos, Adam Warlock, and Eros, as each had been forbidden in Mistress Death's realm --- and is thus no longer bound by rules that govern every other living being. With such power, Thanos gains possession of the galactic regulator --- itself linked to, and guarded by, the Living Tribunal --- and using the regulator and the Reality Gem, absorbs the powers of every galactic celestial being. In his near-complete form, Eros and Pip witness a Thanos possessed of the powers of Lords Chaos, and Order, the entities Love and Hate, the Phantom Stranger, Galactus, absorbed the powers of all the Infinity Gems, and --- finally --- absorbed the personified concepts of Eternity and Infinity. Thus empowered, Thanos takes possession of Mistress Death's force --- itself having been absorbed by his own future self.
Unlike past versions of Thanos that merely utilized the Infinity Gems, this version of Thanos has become one with them, imbuing him with nigh-omnipotent powers that allowed him to take over and embody every other cosmic entity. From his first appearance, Thanos is a massive threat even without the aid of cosmic powers...having absorbed them, he becomes the most powerful being in existence...transcending even Death itself.
