The six Infinity Stones - embodying Time, Space, Mind, Power, Reality, and Soul - are some of the most powerful objects in the Marvel Universe. They have been responsible for staggering misery and destruction, both before and after Thanos used them to wipe out half the universe’s population. However, there is a lost Infinity Stone, and thanks to a variety of reasons, both in-universe and out, this Stone can never be used again.
The Infinity Stones have formed the backbone of epic cosmic sagas, both in the comics and the Marvel Cinematic Universe. While their origin stories vary from medium to medium, the Stones represent the fundamental powers of the universe. Possessing just one of the Infinity Stones grants the user fantastic powers; bringing together all six can make the user a god. Together these six Infinity Stones are one of the most formidable forces in the universe - but once there was a seventh Infinity Stone, and its saga is a fascinating one.
In the early 1990s, Malibu Comics launched a shared-universe imprint called "the Ultraverse." The line initially proved popular with fans, but unfortunately launched shortly before the comics market all but collapsed. Sales began to dwindle, and in 1994, Marvel Comics bought Malibu, essentially making the Ultraverse part of its larger multiverse. Crossovers began almost immediately, and as the Ultraverse began to wane, Marvel sought a way to reboot it. Enter the Ego Stone, the lost Infinity Stone. In the story, the Infinity Stones were created when Nemesis, a godlike being, was split apart. There were the six Stones fans know today, with each representing an aspect of Nemesis’ powers; however, a seventh Stone, which contained parts of Nemesis’ consciousness, was also created, ultimately finding its way to the Ultraverse. When the others also entered the Ultraverse, the Ego Stone began to seek a way to reunite with them.
More and more heroes and villains began crossing over into the Ultraverse; the Black Knight and Sersi of the Eternals among them. Sersi immediately detected the Ego Stone, and when she went to retrieve it, the Stone possessed her, beginning the process of reuniting with the rest. She was successful in bringing them together, returning Nemesis to life; Nemesis then sought to remake both the Ultraverse and the mainstream Marvel Universe. However, the seven Stones could not work together in harmony, which gave the heroes the edge they needed to defeat Nemesis, and the Ego Stone was destroyed, its power divided among the other six.
The act of the Ego Stone’s destruction rebooted the entire Ultraverse, but merely sent Sersi, Black Knight and the others back to their Earth. Unfortunately, nothing could really save the Ultraverse in the real world, and the line was gone completely within two years. Given the legal difficulties of bringing back the Ultraverse, and since the Ego Stone was directly tied to it, there is little chance fans will ever see it again.
How might the Ego Stone have impacted Thanos and others who have used the Stones? Ego Stone users in the Ultraverse often had their own consciousness overwritten by the Stone; likewise, the Stone retained mental imprints of former users, making it a swirl of voices, which hobbled its power. This would have made it useless to Thanos and Adam Warlock, even posing a threat to their plans. The Ego Stone, Marvel’s lost Infinity Stone, is the most fascinating stone of the group, but thanks to legal reasons, it will almost certainly never be seen again.