The Ego Gem is the seventh Infinity Gem that was introduced in the crossover with Marvel's Avengers and Malibu's Ultraforce. When the Infinity Gems were taken into the Ultraverse by Rune, the Ego Gem contacted and took the Avenger Sersi as it's host.
The Ego Gem was created by Terry Kavanagh and John Statema and first appeared in Avengers/Ultraforce Prelude #1.
The Ego Gem hasn't appeared once since the story that introduced it and the existence of it has gone ignored and overlooked by subsequent writers. It is possible that the Ego Gem's disuse is due to a legal issue which prevents Marvel from using Malibu creations. With that said, in Brian Michael Bendis' Avengers books there have been a couple of mentions of there been "seven Infinity Gems", but this could simply be poor continuity.
Major Story Arcs
Loki eventually gained possession of the six gems, which attracted the Grandmaster, who wished to re-obtain possession of the Mind Gem he had lost to Thanos. Together they made a wager and pitted the Avengers and Ultraforce against one another. If Grandmaster won, he would get his gem. If he lost, he would give Loki the Ego Gem.
Loki's trickery prevailed and he was rewarded the Ego Gem which quickly reunited with the other gems and caused the rebirth of Nemesis. Luckily, the sentience each individual gem had obtained since the original split distracted Nemesis just enough to allow the heroes to team up and defeat it, thus splitting into individual artifacts again.
Powers and Abilities
The Ego Gem is the consciousness of the being Nemesis, an omnipotent being who through loneliness ended it's existence, thus forming the Infinity Gems. The only true power of the Ego Gem is the resurrection of Nemesis once reunited with the other six gems.