The Reality Gem allows the impossible to become possible. The gem has the power to completely alter, manipulate and control reality by changing the physical laws of the universe.
The Reality Gem is the most powerful and most dangerous of the Infinity Gems. Special knowledge and care is required to use it's power, and even then it should only be used on a small scale. The Reality Gem should be used in unison with the other Gems, most specifically the Time and Space Gems. The Gem has the power to control as well as destroy reality if improperly handled or used by an inexperienced user.
With the use of the other gems, the Reality Gem can affect the entire universe and bend it to it's users will.
Originally known as the Soul Gems, the Infinity Gems were collected by the Elders of the Universe to destroy Galactus. After their failed attempt, the Reality Gem remained in the custody of the Collector. The Collector traded it to Thanos, who managed to collect all the Gems into the Infinity Gauntlet.
After his defeat, the Gauntlet briefly came into Adam Warlock's possession before he gave the gems to selected people for safe keeping. This team would come to be the Infinity Watch. The Infinity Watch had a secret member whom was entrusted with the Reality Gem - the Mad Titan Thanos himself - the only being knowledgeable enough to know that the Gem's destructive power was to be respected. This choice displeased Eternity greatly.
Thanos only used the gem one time. In anger over his love Death consistently spurning him, he decided to use the gem to create a universe in which she returned his affection. But first, he brought Captain Mar-Vell back to life, who talked Thanos out of using the gem for this purpose. Thanos knew deep inside that he could not use the gem to achieve this false reality even if he didn't destroy the universe trying (which is why he summoned Mar-Vell, knowing he would talk him down).
Wielders of the Reality Gem
In Other Media
Thor: The Dark World
The Reality Stone appears as a major item in Thor: The Dark World, where it is called The Aether. Unlike its comic counterpart, the Aether appears as a liquid like substance rather than a solid stone. It is almost parasitic in nature, requiring a host that it draws life from. Thousands of years ago, Malekith planned to use the Aether to lunge the Nine Realms into eternal darkness, but he was defeated by Bor, who then had the Aether hidden away so that nobody could ever find it.
In the present, an astrological event known as the Convergence causes the walls between the Nine Realms to become weaker, resulting in numerous spatial anomalies. While in London, Jane Foster encounters one such anomaly, which leads to her accidentally discovering a portal to the Aether. It immediately enters Jane's body, causing her to reach out to Thor for help. Malekith soon returns, and attacks Asgard in order to capture Jane.
A long series of events ensue, and during the final battle, Malekith is able to harness the power of the Aether for himself. He and Thor have a heated battle across the Nine Realms, which finally ends when Thor is able to kill Malekith once and for all. In the movie's mid-credits scene, it is shown that the Asgardians had managed to contain the Aether in an urn. Sif and Volstagg travel to Knowhere, where they give the Aether to the Collector for safekeeping, reasoning that it is unwise to keep it back in Asgard, where the Tesseract (the Space Stone) already resides.
Guardians of the Galaxy
In Guardians of the Galaxy, a brief image of the Reality Stone is seen when the Collector explains the origins of the Infinity Stones to the Guardians.
Avengers: Age of Ultron
In Avengers: Age of Ultron, Thor sees a vision of the Infinity Stones that have been found so far, including the Aether. It is shown that the Aether does indeed possess the ability to transform into a solid stone.