Garm

In Norse mythology, Garm was a huge four-eyed dog that guarded Helheim, the land of the dead, living in a cave called Gnipa. It was usually covered in blood. Garm was the greatest of all dogs (excluding the Fenris wolf). In some traditions, he is the dog of the Frost Giants. He will howl and signal the beginning of Ragnarok, according to some eddic material. During Ragnarok, Garm and Tyr will kill each other.