In the Warhammer 40k universe, Soulstones are used by the Eldar to capture their souls upon death, saving them from being consumed by Slaneesh.
The reason they drew the ire of Slaneesh is they were the dominant species in the galaxy in the past and became prideful and decadent. Their extreme debauchery awoke She Who Thirsts from her slumber and brought her back to the universe. Even to this day, she lies in wait to consume Eldar souls.
Soulstones are incorporated into the body armor of every Eldar warrior; they are usually located above the heart. They appear as ordinary gemstones but act as a receptacle for the soul in the event that the Eldar is slain. The stones are collected from the bodies of the slain soldiers and taken back to their Craftworld.
The soulstones are stored in a central chamber, an area held in awe and silent regard for the dead. While their physical bodies have passed on, the power of the soulstones is often harnessed by the Eldar both for mystical purposes and to assist in enhancing their war machines.
An Eldar Farseer can enter the central chamber and commune with the spirits who reside there. By doing this, they are able to foresee the consequences and repercussions that their actions will have on the future. Farseers use this knowledge to assist in guiding the actions of their Craftworld to help better the odds for Eldar survivability. Farseers can also utilize the force of the soulstones to assist with channeling their psychic powers.
The soulstones of fallen warriors are often used to control powerful mecha-like battle units of Wraithguard and the fearsome Wraithlord. The souls of the dead warriors bring their skills from the past to animate these wraith units to assist the living in battle. Eldar vehicles are often outfitted with soulstones. When the crew of the transport is shaken or stunned from incoming enemy fire, the soulstone may assume the functions of the machine, guiding it and firing its weapons, until the crew can return to their capacities.
The Eldar long for the day when Slaanesh is defeated and the soulstones can be broken, releasing the souls imprisoned there for so long.