When the Primarchs came to the World to vainquish the Great Adversary, Nergal, as one of the strongest Primarchs, was on the frontlines. They were outmatched, however, and Nergal was consumed. However, as Primarch of the Soul, he managed to protect his own Soul from devastation and retreated to Irkalla, his body and mind mangled beyond hope. His followers encased him in the Eternal Throne, a construct channeling the souls of the devout to keep Nergal alive, and hopefully one day heal him.
Meanwhile, a group of Primarchs ambushed the Great Adversary once more. This group included Ereshkigal, Primarch of the Flow, and Hasamelis, Primarch of Motion. Though Hasamelis had the power to send the Primarchs away to safety, they all agreed to fight to the bitter end. As a result, though the Great Adversary was greatly weakened, all of them were slaughtered, except Hasamelis himself who disappeared. Upon learning of the passing of his consort, Nergal lost the shreds of sanity he had left and his nightmares started manifesting themselves in Irkalla. Over the millenia, his servants have been tirelessly attempting to heal their Lord, their population slowly decreasing as their Souls were consumed to sustain his ailing body.