Reality Gem

The Reality Gem was one of the [Infinity Gems], yellow in color, which could actually alter reality itself to match the user's desires. It is the most difficult of the Infinity Gems to use, as a user must understand the nature of the gem and its powers beforehand. The Reality Gem was in the hoard of the Elder of the Universe known as the Collector, who could make no use of it but kept it as a trophy. Thanos gained it by trading the (temporarily) de-aged Runner to the Collector for it. It became part of the Infinity Gauntlet.

The Reality Gem remained part of the Gauntlet until Adam Warlock was forced to separate it into its parts, at which point it was given to a secret guardian- who turned out to be none other than Thanos himself. Along with the other Infinity Gems, the Reality Gem was stolen by Rune and scattered across that entity's native reality. Adam Warlock is currently seeking them out.