Úlfhéðinn (plural Úlfhéðnar and derived from ‘úlf’ meaning wolf, ‘héð’ meaning skin to give “Men In Wolf Skin”) is an Old Norse term for a warrior with attributes parallel to those of a Berserkr, but with a lupine aspect rather than ursine; both terms refer to a special type of warrior capable of performing feats far beyond the abilities of normal people.
It is a practice that is dying out as it no longer has real military value in our current society.
Contrary to what I’ve read elsewhere, I do not consider it possession from the spirit of the animal but rather a self-induced trance state which connects the user to a more primal state of consciousness, facilitated by none other than Fenrisúlfr himself.
The process (which I will not describe here) is coded into the story of Fenrisúlfr found in the Prose Edda where he is bound by Gleipnir.
The process involves galdr, breathing exercises, ritual movements and direct communication with Fenrisúlfr so that he might lend the warrior his wild, ferocious power, which the Úlfhéðinn must fetter using methods as described in the creation of Gleipnir, his fettering and his tethering to Gjoll (“Scream Rock”), which are again coded (for example, Gjoll refers to the screams the Úlfhéðinn issues towards the end of the process). The threat that Ragnarök will occur if the fetter is broken is a very real warning to the Úlfhéðinn.
The cost to use this power is also expressed in the lore and represented in the sacrifice of Tyr. It should be clear that this is a practice of war and, of course, Tyr is the god of war.
It’s also worth noting that in stanza 40 of Völuspá when referring to the brood (a brood of warriors) of Fenrisúlfr there will come “a moon-snatcher in troll’s skin.”, where being “in troll’s skin” is also found as a reference to Úlfhéðnar in other lore such as Egil’s Saga and again shows the real connection between the Úlfhéðnar and Fenrisúlfr, rather than a totem animal.