Aether
The Aether is the liquid form of the Reality Stone, created by the Dark Elves as a way of weaponising the Stone.
It was used during by Malekith the Accursed during the First and Second Dark Elf War, and was briefly absorbed by Jane Foster.
Although it was intended to make the Stone useable, it remains extremely dangerous in its Aether form, even to powerfully magical beings such as Dark Elves and Asgardians.
Following the defeat of Thanos, the Aether was scattered along with the other Stones, and it's location is now known only to Vision.
Original Timeline
In the original timeline, the Aether was given to the Collector by Odin following the Second Dark Elf War. It was stolen from the Collection by Thanos, who used it to prevent Gamora from killing him.