Mirai otherwise known as the Black Pit, is the Elven Underworld. The dark realm of Mirai is ruled by Ereth Khial the Pale Queen. The way to get in and out of Mirai is through the a door, referred to as the Last Door, which is guarded by Nethu. It is Nethu who prevents wayward souls from escaping, and the living from entering and stealing the secrets of the dead.
However, since the Coming of Chaos, most Elven souls don't reach Mirai and are claimed by the Chaos God Slaanesh. Slaanesh desires Elf souls above any other, and is always hungry for more to devour. Sadly most Elves souls throughout history, are consumed by 'She Who Thirsts' as the Elves refer to Slaanesh. This horrible fate terrifies all Elves, so in order to avoid being devoured by Slaanesh, the High Elves bind their souls to waystones. So when their ultimate death finally comes, their souls will instead be used to empower the magical wards protecting Ulthuan. Whenever High Elves are far from Ulthuan, they will bring and wear stone necklaces so that in case they meet their fate their souls will be captured by it and not by the Chaos God Slaanesh.
However not all unprotected souls are doomed to be consumed by the Dark God, on occasion the Trickster God Loec will rescue souls by challenging Slaanesh in a game of chance, and then of course cheating to win. In addition to Loec, Ereth Khial often tries to send her winged servants to steal souls from Slaanesh's grasp, and sometimes if done quick enough succeeds. Even though these rarely happens, even the smallest chance to avoid their souls from being consumed by Slaanesh, give the Elves some reminisce of hope.
Even if their souls are saved from being consumed by Slaanesh, they will receive no paradise, for their souls often are faced with an eternity as slaves within Ereth Khial's Army. Even though it is frowned upon for High Elves to do so some Elves will worship The Pale Queen just for this reason, hoping to earn her favor. Ereth Phial, uses every soul she receives to serve in her army for the day Rhana Dandra comes. Rhana Dandra is the Elven apocalypse. It is in this day that the Cadai and the Cytharai Gods will have a final battle and Ereth Khial's Army of fallen Elven souls will be pitted against Asuryan's Creatures. It is also in this day that the Elven Gods will also battle the Chaos Gods, be the outcome good or ill.
- Warhammer Armies: High Elves