Úlfhéðnar are well beyond Berserkir. They are known as Óðinn's special warriors, and elite Viking forces. They are metaphorically associated with wolf pelts (in contrast to regular Berserkir who are associated with bear pelts), as they are known 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. Úlfhéðnar are exclusively males.
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.
The Church, however, has always had an unholy fixation on both Víkingar and wolves. So the clergy quickly claims Úlfhéðnar (and Berserkir as well) are "Heathen Devils". When Eiríkr Hákonarson (son of Hákon Sigurðarson) decides to embrace Christianity, he therefore outlaws Berserkir and of course, Úlfhéðnar as well. With the Christianization of Iceland, Grágás (medieval Icelandic law code which contained a Christian law section) also sentences Berserkir and Úlfhéðnar to outlawry.
Úlfhéðnar nowadays are still stigmatized. Some of the Úlfhéðnar's traditional skills are seen by western societies as "toxic masculinity", while Úlfhéðnar's spirituality, knowledge, and way of life are often demonized by the church as well as feminized societies.
Úlfhéðnar today remain nonetheless super crazy víkingar (really) and modern elite warriors. They are masters of all things Norse. They've got the spirituality, the mental, the physical fitness, the knowledge and the skills (ranging from survival and martial, to healing, or even the ability to reach altered states of consciousness).
Úlfhéðnar are born. Not made. It is therefore a matter of developing underlying potential in the very few males who've got what it takes in their blood, genes, and spirit. These dormant Úlfhéðnar are normally selected during Berserkr training, or within specific military units and special forces, from the best among their peers. Úlfhéðnar can also be identified during any stage of training and developed accordingly as applicable.
Úlfhéðnar see wolves as sacred, and as a result, never hunt them, or even take photographs of them. Wolf pelts are gathered from individuals who experience a natural death.
While Úlfhéðnar are typically associated with Óðinn in popular culture, their ássinn is either Týr or Þórr (there's a great deal of flexibility for such things).
Úlfhéðnar Hirðir are active combat-ready and special-forces-level units, composed of 12 to 18 men, led by two Hirðarhöfðingjar (Unit Commanders), who are always Úfhéðnar. The Hirðarhöfðingjar are responsible for the leadership, training and readiness of their Hirð, and ultimately, Hirðir under their command. They are also responsible for the resolution of disputes within their own unit, as well as for all actions and activities involving the members of their own Hirð.
Úlfhéðnar Hirðir fall under the direct command of another Úfhéðnar Hirð with dominant status within the organization.
Hirðarhöfðingjar of the dominant Úlfhéðnar Hirð are also Hershöfðingjar (Force Commanders), acting as commanders of Norðsherstjórn (Northern Force Command, or NHS), and are therefore in command of multiple Úlfhéðnar Hirðir as well as Berserkir Hirðir. They are also ultimately in command of Jómsvíkingar as well as Skjǫldrinn.
An entire chapter of “Úlfheimr: The Tenth World” is wholly dedicated to Úlfhéðnar, covering origin, history, structure, connection to Jötnar and wolves, relationship with Blendingar as well as Berserkir, and more.