The Rune Priests are fucking Librarians of the Space Corgis, and their existence is one of the most infuriating for those who despise the fucking yiffers. Part of the reason why is because of the notion that the closet furries have a particular hate boner for psykers, especially the Thousand Sons, and are known to treat psykers with suspicion and mistrust. As you can imagine, during the burning of Prospero, Magnus pretty much went, "What the absolute Fuck on Tizca's Tip, Russ! You hate our people for who we are and yet you yourself use psykers in your ranks!?", to which Leman promptly rebutted, "You betrayed the Emprah! FUCK YOU!""
The thing is, the Rune Priests use shamanic rituals to avoid being sucked into the Warp. Totems and runes allow the Rune Priests to exercise caution and precision with their powers, but they know damn well to watch out for psychic predators and know their fucking limits. The Thousand Sons are reckless and self-indulgent with their powers. They consorted with daemons even before Magnus singlehandedly shattered the Emperor's design to utilize the (much safer) Webway. The Wolves don't hate the Thousand Sons because they're psykers; they hate them because the Vlka Fenryka had been fighting monsters and uncontrolled psykers since Russ was found, possibly including the two missing Legions, and the Thousand Sons gleefully charged down the path to damnation while treating the Space Wolves and everyone else like idiotic barbarians, and then acted like victims when their hubris and Tzeentch conspired to bite them in the ass.
Even so, the rituals and such used by Rune Priests means they're sorcerers. If you manipulate the Warp in a way that is not just a natural connection to it that your soul possesses, then it is sorcery. Which is illegal. Just because it uses decorated beast bones and shaking carved sticks instead of chanting in mystical tongues and waving your fingers doesn't make it any less sorcerous. Rune Priests are allowed to stick around because they claim that their powers come from the World Soul of Fenris, like the World Soul of Chogoris that its Stormseers use. The difference is that Fenris is not confirmed to have a World Soul like Chogoris, and the Fang is covered with the screaming faces of dead people and the wolf spirits summoned by Rune Priests are brought from the Warp... making them daemons. Also unlike the Stormseers, the Rune Priests use the Warp directly for their powers, whereas the Stormseers tap into their connection to the World Soul of Chogoris to filter their psychic power to keep it clean of Chaotic taint AND to protect their minds and souls from Warp predators.
Anyways, like the Librarians of Codex Chapters, Rune Priests are combatant psykers, using their psychic abilities to aid their brothers on the battlefield; off the battlefield, they are the keepers of the chapter's librarium and store of knowledge.
NOTE: Rune Priests also exist in Warhammer Fantasy, specifically as a class in the video game Warhammer Online, representing the adventurer version of Runesmiths of the Dwarfs, which is much less hypocritical for a Chaos power-hating faction than the Space Corgis by the virtue of being given to them by their God Empress Of Dwarfkind.
Rune Priests preserve their Chapter's entire history not in written form, but memorize it in great sagas. A Rune Priest who begins his training as a Skald is usually assigned to a single Great Company where he is taught its tales. With every new Great Year he joins another Company until he has learned the Chapter's full history and can be sent to whichever Rune Priest needs an apprentice.
The basic weapons of the Rune Priests are their psychically charged Runic Weapons. A common power among Rune Priests is the ability to summon storms across the battlefield which can cover the army's advance. Furthermore, they can cast runes which predict the ebb and flow of future events. These runes are usually carved from the bones or teeth of totem animals.
Many Rune Priests are psyker-linked to cyber-ravens, which are known as "Choosers of the Slain" to the Space Wolves. Their link enables them to see through the ravens, eyes and control them, thus allowing them to use them as scouts and messengers.
Warp vs Fenris
The major difference between Rune Priests and most other Librarians is that they do not believe their psychic abilities are derived from the Warp, but rather from the animistic spirits of their homeworld, Fenris; which is channeled through totems and runes on their armour.
There does seem to be an element of in-universe discrepancy on this: In Thousand Sons Ohthere Wyrdmake and Leman Russ say repeatedly that the Rune Priests did not use the power of the warp, even while accusing the Thousand Sons of corruption at the Council of Nikaea. However, in Prospero Burns Helwintr and Russ don't even mention this bullshit. The Space Wolves are not the savage ignoramuses that they want everyone to believe they are and seem to have a perfect grasp of the situation and can clearly differentiate between psyker and sorcerer, which is conveniently compatible with what will happen after the heresy. But the Thousands Sons seem to fall for it, so in their eyes, that fact that the Wolves would utter this kind of stupid nonsense is somewhat believable. None of which changes the fact the Space Wolves have either deluded themselves or are lying to everyone else about where their own psychic powers come from, making them either idiots or hypocrites respectively.
The discrepancy between the depictions of "ignorant savage" vs "lying hypocrite" is addressed somewhat in Wolfsblade. The Rune Priest traditions were descended from the shamanistic rituals of Fenris's native tribes, and have remained unchanged for countless centuries. Russ claims that his Rune Seers do not draw on the warp; that they (along with the Stormseers of the White Scars) use "older traditions" which shield themselves from corruption from the warp by limiting the power they can draw from it directly; they acknowledge both the power of Fenris and the Warp side-by-side, essentially making their culture a lens through which the Warp can be viewed comparatively safely. They seem to just be using Fenris as a totem for channeling the Warp.
Because of Russ' stance on his Rune Priests, he is flat out called a hypocrite by Rogal Dorn (who coincidentally was the only Primarch who still adhered to the Edict of Nikaea at that point) who points out that there are no limits on power, no matter how you slice it. Later Russ begins to wonder if he really was a hypocrite, and privately resolves to examine his Rune Priests' relationship with the warp on another day. Whether he actually did this or not is anyone's guess.
Russ himself takes a jaunt into the Underverse (essentially a sanitised version of the Warp) to entreat with the Wights to provide a means for wounding Horus. He succeeded because he understood and respected the symbolism of the realm. One might say that the Underverse is exactly the same as the Warp, but the Underverse comes with rules and conditions to prevent travelers from wandering astray ("Drink, but don't eat the meat" etc) and the wights left him alone at least until he addressed them. It is not the first non-Chaos sub-realm to appear. In the alternative, if he had travelled directly into the warp he would have been subject to all of the crazed logic of Chaos and likely would have had a considerably more violent experience; while his exposure to the Underverse was fraught with threat and difficulty, he passed through relatively unmolested.
|Forces of the Space Wolves|
|Command:||Wolf Lord - Wolf Guard|
|Priesthood:||Iron Priest - Rune Priest - Wolf Priest|
|Troops:|| Blood Claw - Fenrisian Wolf - Long Fang - Skyclaw |
Swiftclaw - Thunderwolf Cavalry - Wulfen - Grey Hunter
|Vehicles:|| Bike Squad - Rhino - Razorback - Land Speeder |
Predator - Vindicator - Whirlwind - Space Wolves Dreadnought
Land Raider (Land Raider Crusader
Land Raider Redeemer - Wrath of Mjalnar)
|Flyers:||Stormfang - Stormwolf|
|Allies:||Space Marines - Primaris Marines|