Marvel's Thanos has sought out some of the most extraordinary powers in the known universe to enact his plans of pleasing Death and becoming the most powerful villain in the Marvel Universe. From the Infinity Gauntlet to the Cosmic Cube, the Mad Titan's obtained reality-shaping weapons and used them to terrorize Earth's Mightiest Heroes. However, the most powerful object Thanos acquired, the Heart of the Universe, was used to save his reality from fading out of existence.
Marvel: The End by Jim Starlin, Al Milgrom, Christie Scheele, Heroic Age, and Dave Sharpe told the story of the final days of the Marvel Universe. In the story, the Avengers, X-Men, and other heroes are killed by an omnipotent being named Akhenaten. Thanos learns of the source of the Pharoah's omnipotent powers, the Heart of the Universe, which the Celestial Order staunchly guarded. After Thanos and the Defenders distract the Celestial Order guards, the Mad Titan dives into the Heart of the Universe and gains its incredible abilities.
The Heart of the Universe gives Thanos powers beyond any Infinite Stone or cosmic artifact. The villain uses his powers to reshape the universe and create different realities. Thanos compares the Infinity Gauntlet's power to the Heart of the Universe, noting the former gave him control of mastery of time, reality, space, power, and mind, but it was all external - while he's bonded to Omni-reality with the latter. He is everyone and everything, all at once. Thanos evaporates Eternity, kills a Celestial, and destroys the Living Tribunal and all of those threatening his reign.
However, since he used his powers without thought, Thanos absorbed the entire universe - leaving himself in a void of nothingness. It takes Adam Warlock (who was outside of space and time during the universe's destruction) to convince Thanos that infinite darkness isn't the end he wants. Thanos ends up using the Heart of the Universe to restore reality to its past self, while sacrificing himself and his great power to make it happen.
As has happened before, Thanos went mad with power. Only this time, his newly acquired omnipotence led to him absorbing all that's ever existed in his reality and landing himself in a void of darkness. With the Heart of the Universe, Thanos was not only the strongest he's ever been, but he was more potent than any villain before him. Marvel Comics even had to note the story was non-canon after it was published because of how insanely strong the Mad Titan got. In the end, Thanos' ultimate form was unimaginably strong - but was even too much for him to handle.