The Erlking is a Fenrisian deity known as the lord of wights and death, or Nettagangr. The Erlking oversees the Muspjall, the hall of those who die a coward's death of old age. His halls consist of savage Wulfen feasting on barely-cooked human meat like barbarians.
During the Horus Heresy, Leman Russ journeyed into the Warp and encountered the Erlking, who gave Russ a series of challenges to complete in exchange for information on how to wound Horus. The first challenge was to drink the damned soul Amarok's bowl dry, next to wrestle an old crone, and finally move the Erlking's great sleeping wolf. Russ failed in the first three, and his soul rested on completing the fourth which was to explain what the Erlking challenged him to complete. Russ explained that the first challenge symbolized the changing of the seasons on Fenris, the old woman represented the inevitability of age, and the unmovable wolf inescapable death. The hall crumbled, replaced by a finely dressed, civilized version of Russ that was revealed to be his fate had he never landed upon Fenris. This Russ revealed the nature of the Spear of Russ, that it held a portion of the Emperor's power and could illuminate the truth to those it pierced. With that, the false-Russ impaled the Wolf King, revealing to him the truth regarding the nature of the Primarchs. Russ despaired, and the false-Russ revealed that this knowledge would one day see him leave Fenris.