Wulfen

Within the gene-seed of every Space Wolf coils the Canis Helix, the strand that gifts the Chapter its predatory senses and its kinship with the great wolves of Fenris. In some warriors that inheritance runs wild, dragging the man back down toward the beast until he is transformed into a Wulfen, a hulking, fur-matted horror of claws and fangs that fights with the mindless savagery of a cornered animal. This descent is the Chapter's oldest and most closely guarded shame.

In the days of the Great Crusade those touched by this curse were gathered into a single formation so that their ferocity might be wielded as a weapon and contained for the safety of the Legion. In time that company vanished utterly, struck from the Chapter's records and remembered only in hushed and fearful myth, so that to speak openly of the lost brethren became forbidden among the Sons of Russ. For long ages they were held to be gone forever, a wound the Chapter would not name.

Then, in the Imperium's hour of greatest peril, the Wulfen returned from the mists of legend to fight once more against the servants of Chaos, and their reappearance was hailed by the Chapter's priests as a sign that the primarch Leman Russ himself would soon come again. Yet their homecoming is a double-edged blessing, for the curse they carry has grown contagious, and the mere presence of a Wulfen may now stir the beast in brothers who had thought themselves free of it.