A monster outside but a knight inside (слэш)

There was this old tale running through the air of nine realms, a story about an old beast hidden among the shadows of Svartalfheim. A beast that roars and howls could be heard at night. A monster that had dark fur with crimson paws and fangs covered by the blood of its enemies. A beast that was still undefeated, an animal that killed anyone on its way.
And now it was about to kill Loki.