The Ulfhednar Wolf were a group of Viking warriors who originated from ancient Norse religious rites (most likely those involving Odin, chief Viking God who was most venerated by the Vikings, had two giant wolves as pets, and was destined to be swallowed by a giant wolf at the battle of Ragnarok). The Ulfhednar wore wolf skins, and their own skin was black-died. These howling warriors were so wolf-like in behavior and ferocity that they inspired/influenced later European werewolf folklore.
Like the berserkers, they performed chants and ritual prior to battle to get in a “Berserker Rage”, when, through adrenaline, they became much stronger (possibly upwards of ten times stronger than normal) and faster, became immune to pain, and bleed less.
Úlfhéðnar are well beyond Berserkir. They are Óðinn’s special warriors, and elite Viking forces. They are known to wear a wolf pelt (in contrast to regular Berserkir who wear a bear pelt), and to be inhabited by the spirit of wolves. Úlfhéðnar are capable of performing feats far beyond the abilities of other warriors. Úlfhéðnar are hamrammir (shape shifters). They don’t wear a helmet or a mailcoat, bit their shield in a rage prior to each battle, kill enemies with just one blow, and are immune to fire or iron.
Without Berserkir and Úlfhéðnar as shock troops, King Haraldr hárfagri would have been unsuccessful in his quest to unify Nóregr (Norway). After all, Úlfhéðnar, were not only the best warriors of the time, they were also the most feared.
It was said that to kill an Ulfhednar, you had to crush their heads.