Reality Stone

"Their leader Malekith made a weapon out of that darkness and it was called the Aether. While the other relics often appear as stones the Aether is fluid and everchanging. It changes matter into dark matter. It seeks out host bodies, drawing strength from their life-force."

- Odin to Thor and Jane Foster

The Aether (pronounced ēTHər) is an Infinity Stone, the remnant of a singularity that has existed since before the universe began. A force of ancient and catastrophic power, the Aether has the potential to transform the entire universe. This "stone" is unique since it is in the form of a dark, red, viscous liquid, rather than a literal stone, like the other five. However, evidence from the Collector's Museum, suggests the Aether may not have always been this way and may have originally been a stone similar to the others. It is currently located in the Collector's Museum. The Aether represents Reality.

Origin
"Long before the birth of light there was darkness, and from that darkness came the Dark Elves. Millennia ago the most ruthless of their kind, Malekith, sought to transform our universe back into one of eternal night. Such evil was possible through the power of the Aether, an ancient force of infinite destruction."

- Odin

The Aether is a remnant of one of the six singularities which existed before the universe. After the universe came into existence, four cosmic beings known as the Cosmic Entities forged the energies of these singularities into six immensely powerful objects called the Infinity Stones. The Aether represents reality, as it possesses the power to bend the very laws of reality and physics, converting matter into dark matter. it is unique in that instead of taking the form of a stone or solid object, it appears in the form of a flowing liquid or mist.

The First Attempt
"Sire, the Aether, shall we destroy it?" "If only we could, but its power is too great. Bury it deep. Somewhere no one will ever find it."

- An Einherjar soldier and Bor

The Aether was used prominently by the Dark Elves of Svartalfheim, in particular, by their leader Malekith. Every five-thousand years, a cosmic event called the Convergence causes the Nine Realms to become aligned. In 2987 BC, Malekith planned to use this event to release the Aether and plunge the nine realms into eternal darkness; however, the Asgardians intervened and took the Aether from the Dark Elves by force, watching as the race was apparently decimated in battle. Bor then had the Aether hidden away where no one could find it, and over the following five millennia, its story and location faded from known history.

The Second Attempt
"The Aether cannot be destroyed!"

- Malekith

In 2013, near the beginning of the Convergence, a portal opened up to where the Aether was located; Jane Foster happened to walk through this portal and, through curiosity, made contact with it, causing it to become active and infest Foster's body, using her as a host. After arriving back on Earth, Foster exhibited an immensely powerful energy projection when being touched on the arm, and was taken to Asgard by Thor where the Aether was discovered by Odin. During this time, Malekith sensed the Aether's activity and attempted to retrieve it by attacking Asgard. In the end, through the sacrifice of Frigga, Malekith and his army left before he could obtain the Aether. Later though, when access to the Bifrost Bridge had been restricted, Thor, with a dying Foster, got Loki to take them through a hidden passageway to Svartalfheim. After getting there, they confronted Malekith and through various circumstances, the Dark Elf drew the Aether from Foster and into himself and left the planet. Both parties then arrived on Earth, where Malekith used the Aether to battle Thor across worlds during the Convergence until eventually destroying Malekith as the borders of the nine realms are blurred and anything can happen. Afterward, the Asgardians were able to contain the Aether, which was later given to the Collector for safekeeping.

A Vision
Thor had a vision of the Infinity Stones and their role in the events of the previous years, the Aether included. It is revealed that the Aether can indeed take the shape of an Infinity Stone rather then being just a liquid form.

Aether Energy
"What I could do with the power that flows through those veins."

- Loki

The Aether converts matter to dark matter, and seeks out host bodies in order to draw strength from their life force, energy from living organisms, thereby sapping them of energy in a process that would inevitably prove fatal. As used by Malekith, the only one who fully understood its capabilities, it was capable of creating tendrils of itself to strike out at opponents, or even regenerating lost limbs, along with creating a tornado of Aether energy. Whilst bonded to a strong host, Malekith, the Aether greatly enhanced his physical strength and durability thereby allowing to fight Thor on a reasonably even basis. On its own, the Aether is capable of sending out shockwaves fillied with its energy and pulses of power that can throw back any who it deems a threat to itself. These blast have been enough to easily stun beings as physically powerful as Asgardians. The Dark Elves and Malekith appeared to have a connection to Aether in that it awoke them from hibernation when the Aether was used again and Malekith could sense the Aether's location.

Trivia

 * Marvel Studios president Kevin Feige has confirmed that the Aether is the "Reality" Stone of the Infinity Stones.
 * In the comics, the Collector owned the Reality Gem before Thanos took it from him.