As part of his quest to obtain the Infinity Stones, Thanos raided Knowhere in search of the Aether. He found the Infinity Stone and presumably deemed the Aether too powerful to control, so he solidified it into the Reality Stone with his Gauntlet. He used the stone against his enemies.

How did Tivan get the Reality Stone?

In 2013, two Asgardians, Sif and Volstagg, arrived at Tivan's museum, bringing with them the Infinity Stone known as the Aether, which had been recovered by Thor after his battle against Malekith on Earth. They gave it to him believing that it would be safe from other parties.

What is the Aether stone?

The Reality Stone (Aether)

The Reality Stone grants the user power to manipulate matter. In Thor: The Dark World the villain Malekith (Christopher Eccleston) wants to use this red liquid stuff called Aether to turn matter into dark matter because he's evil…and do you really need a better reason?