Wulfen
Wulfen are DIGGANOBZ! those Space Wolves who have succumbed to the Canis Helix inside them and gone partially feral. The result is the same whether this is due to lack of self discipline, going berzerk in battle, or wolfing out to avoid chaotic corruption (that's how the Wolves can spend millenia in the warp and not get cocorrupt: Wulfen are almost totally immune to chaotic corruption). Many of the Wulfen were members of the Thirteenth Great Company that went into the Warp by the tail-end of the Horus Heresy with the intent of hunting down and destroying the Thousand Sons and Magnus the Red. Though many hunts were established by their brethren that remained in the material world to find the lost (and Russ), it was eventually concluded that they were lost to the chapter.
The vacancy has since been left unfilled by the annals of Fenris, and their company a symbol of those lost in action.
Back in the Day...
These guys first appeared in the Eye of Terror Codex back in 3E, where they formed their own mini-armylist as the 13th Great Company. Though they lacked the fancier weapons their modern kin had, they made up for it by being absolute savages in combat. They had amazing speeds for on-foot models (and this was pre-Battle Focus Eldar)
As for why they were there? Well, they weren't absolutely confirmed for existing, but there were rumors on the battlefield that they were alive, prompting Logan Grimnar to visit Canada Cadia.
They would sorta remain in the armylist for another two editions with the Mark of the Wulfen upgrade, allowing for some models to eschew any weapons for absolute brawling mastery and immunity to morale.
...And Now
Their comeback in 7th Edition with Curse of the Wulfen is one that made a rather average-tier army with the coming of Decurions back into a great melee army, and these guys were a centerpiece for it.
For 30 points each, these bastards come with a disappointing BS2 but S5, 2 Wounds, 3 Attacks, all that attack at Initiative 5, and get Bulky, Feel No Pain, and Rage in addition to their basic rules. Though they only have a 4+ save, their mobility (as well as FNP) more than makes up for it by allowing them to run and charge at the same turn (with re-rolls to charge). Making a Leader adds +1 Ld and Attack, same as any other.
While Wulfen are melee monsters, their weapons do give them a chance to fight a specific enemy. While they're modestly effective against mobs with their basic CCWs, they can grab some weapons to boost their effectiveness. Stormgrag Auto-Launchers are cheap add-ons that give the Wulfen a ranged attack at the strength of a flamer. Frost Great Axes are two-handed Power Axes +3 (Making them S8 AP2) that can ignore Unwieldy on the first turn of a battle, making them beastslayers. This weakness of Unwieldy is offset by a special rule the Wulfen get that allows them to pile in and attack on the initiative step they are killed, even if they already rolled their attacks. In the case of the axe, this guarantees that they'll always have a chance to hit. Frost Claws are S+1 AP2 Lightning Claws, which gives them remarkable reliability in damage, which is a must since Leaders are stuck with these weapons. They can also grab a Thunder Hammer and Storm Shield, which is incredibly expensive, but gives them both an insanely powerful weapon and a kickass Invulnerable save.
However, the most standout thing they can add is The Curse of the Wulfen, a rule that makes all Space Wolves models that aren't vehicles or wolves/servitors gain some benefit based on what they're doing. Every turn, you need to roll a dice. Every Space Wolf within 6", every Blood Claw/Skyclaw/Swiftclaw within 12", and every Long Fang within 3", gets a result based on one of two tables: Those not in combat use the "Hunt" table, while those in combat use the "Kill" table. Hunt gives them either HoW and re-rolling charges, additional moving distance, and Furious Charge with d3 bonus moves on the charge (unless it has Rage). Kill gives them +1 Initiative, re-rolls to wound in combat, or gain the ability to pile in and attack on the turn they are killed in combat.
When 2-5 of them get fielded as a formation (the Wulfen Murderpack), they get the ability to make a bonus attack if they roll a 6 to-hit (though no further ones) and add +1 to their Curse Tables, allowing for a new effect to allies on the "Hunt" and "Kill" tables: "Hunt" lets them make a move out of the movement phase without penalty, while "Kill" gives a bonus attack.
Warzone Fenris: The Comeback
Curse of the Wulfen
It would be Harald Deathwolf who'd be the first to meet the Wulfen in the new retconned continuity of Post-4E. He and his company were on a mission to save the hive world of Nurades from daemonic infestation when they found themselves ambushed by Tzeentchian trickery. This proved devastating, and they were ready to die warriors' deaths, but suddenly the Wulfen show up. They mop the floor with the daemons and the Wolf Lord meets with them, learning that they are indeed the lost Great Company.
The Wolf Lords soon returned homeward, to Fenris, to debate what portent this meant and how to manage a recent rash of warp storms in their region. The Wulfen with Deathfang only insisted that more of his kind were there. Though none of the Lords could come to a suitable agreement as to their purpose and Bjorn was too busy asleep to answer, they all agreed that one thing was certain: They needed to retrieve the Wulfen before anyone else did and began drawing conclusions. In the meanwhile though, the Wulfen would remain in the Fang to get themselves re-armed. Once properly equipped with the wall-decorations of the Fang, the Wulfen rejoined Deathwolf and the Great Companies began their little scavenger hunt.
However, as the Wolves were hunting, so too were the Dark Angels. After losing contact with some scouts at Nurades, the chapter eventually discovered their deaths and recovered some tainted relics they hid there ages ago (all of this completely unrelated to the Wolves). The Inner Circle "learn" (courtesy of the Changeling) that they were murdered by the Wulfen.(In reality, they fought alongside the Wulfen against Chaos forces.) Though leery, they decide to first ask the Wolves about this situation before deciding to outright assume heresy.
Similarly, the Grey Knights began looking at these new warp storms with concern and began sending out squads to stem the daemons from running loose. Though some of these sites they would find partially cleaned of daemons by the Wolves, Arvann Stern would come across one of the Wulfen and decided it was time to talk with Grimnar.
On Tranquilitus, Wolf Lord Sven Bloodhowl would find a detachment of Ravenwing led by Sammael battling daemons and began teaming up while trying to find the Wulfen. The issue here was that the Wulfen got stasis-bombed. Before Sven and Sammael could come to blows over the giant werewolves, however, more daemons surged forth. This provided the Wolves with their chance to bail, leaving Sammy and his crew to get nearly wiped before falling back. From there, he (or more precisely, the Changeling, disguised as him) concludes that something's seriously fucked up and reports to Azrael.
Sternn would find Grimnar on Vikurus. Grimnar gathered with him his finest guys and MURDERFANG the CurseMURDERBORN in hopes that the Wulfen dread would help find the Wulfen. In the midst of the battle, the Knights would intervene. Sternn insisted on Grimnar's surrender, Grimnar tells them to wolf off. They eventually (again) find a welcome distraction when Stern reveals that his astropaths recovered a message from the Fang that warp storms were approaching Fenris. In a rush, Logan recalls every company back and asks that the Wulfen matter gets shelved until the matter is resolved. However, as they returned, reports began flooding in about marines growing ever-restless, violent even. The Curse of the Wulfen began taking hold.
They arrived to discover that it wasn't the Thousand Sons here to enact vengeance, but instead the Alpha Legion. The Wolves, each with a pack of Wulfen, would deploy on the worlds of the Fenris system, while he would bring the Grey Knights with him. All the while, Krom Dragongaze, indignant at his defeat on Alaric Prime (setting of the Sanctus Reach Warzone), had to keep watch of the Fang when he realized that Bjorn was secretly psychic and was protecting the Fang from Daemons while being crippled. And if it sounds completely stupid, that's because it is. Oh, and Bjorn also warned Krom that the Grey Knights were walking into a trap.
Krom managed to get to Stern before it was too late, and together they battled a Daemon Prince.
It was at this time that the Dark Angels finally burst onto the scene, their fleets supported by those of a dozen other chapters as well as the Navy and Knightly Houses and the motherfucking Rock! Azrael summoned them specifically for the purpose of saving the Wolves from whatever damnation they were approaching. One run-in with the Changeling later, the DAs realized they had been tricked and retreated...just in time for the Thousand Sons to show up.
Possible Real Life Reference
Believe it or not, but the Wulfen may actually be a reference. See, back in 1692, there was this guy called Thies of Kaltenbrun, AKA The Livonian Werewolf. Basically this guy told the church that he was a werewolf, and 3 times a year, he and his kind would become a wolf, go down to hell, and fight the devil and witches. Afterwards, he and his kind would bring back any livestock or grain that the devil and witches had stolen. Of course, the guy was declared a heretic for obvious reasons. It is quite possible that the wulfen are a reference to the idea of turning into a wolf and going into hell to get something back. In the case of the Wulfen, it's hunting down Magnus.