In WH40k, it was prehistoric shamans from all over the world noticing their souls were being eaten by demons and preventing reincarnation, so they all kill them

In WH40k, it was prehistoric shamans from all over the world noticing their souls were being eaten by demons and preventing reincarnation, so they all kill themselves at the same time, merge their souls in the warp, and reincarnate as a single super human.