While Thanos has acquired the six known Infinity Stones in the Marvel Universe to wipe out half of all life from existence, a secret seventh gem has always eluded him: The Ego Stone (also referred to as the Ego Gem). Marvel Comics revealed that there was secretly a seventh Infinity Stone created with the other six powerful gems hidden away in a different reality. The Ego Stone is tied to one of the oldest Marvel beings, Nemesis.
The Infinity Stones are ancient cosmic gems that give the wielder a special control of the Marvel Universe. The original six gems consisted of the Time Stone, Space Stone, Reality Stone, Mind Stone, Power Stone, and the Soul Stone. Thanos famously retrieved all six gems during the Infinity Gauntlet saga, where he used their incredible power to erase half of all life from existence to please the living embodiment of death. However, secretly, there was a seventh Infinity Stone that Thanos never knew about that ties back to the origin of the Marvel Universe and a cosmic force named Nemesis.
While the seventh Infinity Stone might have been introduced in the Ultraverse, it's tied to a cosmic being rooted in the main Marvel Comics universe. In Silver Surfer #45 by Jim Starlin, Ron Lim, Tom Christopher, Ken Bruzenak, and Tom Vincent, Mephisto explains to Thanos that he doesn't know all of the universe's secrets despite acquiring the Infinity Stones. He explains that the gems were once an omnipotent being (who would later be revealed as Nemesis), who was the only living thing in existence. Before killing themselves due to loneliness, Nemesis created life, beginning with Mephisto and demons. After realizing their error, Nemesis self-destructed and split into seven Infinity Stones. However, the Ego Stone would end up in the Ultraverse.
The Ego stone contained Nemesis' consciousness, where whoever used its power in the Ultraverse was taken over by the cosmic being. Unfortunately for the Eternal Sersi, she became Nemesis' vessel as she tried to use the Infinity Stones to remake two universes. However, the Infinity Stones weren't working in tandem and didn't band back together with their maker as she planned, allowing Black Knight and the Avengers to destroy the Ego Stone and get rid of Nemesis for good.
Ultimately, Thanos never got a chance to have an audience with Nemesis nor contain the powers of the secret Ego Stone. However, considering his already considerable might, it's probably best for the Marvel Universe that he never gained the powers of the seventh Infinity Stone. Although, it would be fun to see Marvel Comics revisit the Ego Stone in the future and tell a "What If...?" story imagining Thanos getting his hands on it and merging with Nemesis. Thanos with seven Infinity Stones possessed by Nemesis, could have been one of Marvel's strongest villains.
