Njal Stormcaller, also called Njal (pronounced nyal, or Niel, it is the old norse version of Neil and what they use in the sagas for people like Neil of the Nine Hostages and the Mac or Ui Neils) Stormsson (Nigel to his friends) or the Tempest That Walks, is one of the most powerful Rune Priests of the Space Wolves Chapter, and possibly one of the most potent human psykers period.
Prior to becoming an Astartes, Njal was a regular Fenrisian tribes
manboy. However, during one of his tribe's migrations, they were attacked by a rival clan. He leaped into the enemy boat and annihilated them, but he was speared through the heart for his trouble. He would've died from the wound, but the Rune Priest Heimdall took him to the Fang and made him a full Astartes, training him to use his (apparently latent) psychic abilities.
Njal has also killed a bloodthirster with his psychic powers despite the collar of Khorne granting immunity from psychic attacks, which means Njal probably skullfucked it in combat, putting him on another level in terms of combat power (with Biomancy) possible only Mephiston can match.
He also has Runic Terminator Armour (that he alone can make), though the significance of this has been lost by 7E and he now has normal Terminator Armour.
There could be some confusion between this Njal and another psyker with the same name - the short story anthology, Into The Maelstrom, has a short story called In The Belly Of The Beast featuring Nyal as a scout.
While he and his friend Sven, trainee Wolf-Priest, were led to Sergeant Hakon and in the Rogue Trader Hauptman's ship Spiritus Sancti, they encountered a Tyranid hive ship and boarded it with a torpedo. Nyal's psyker abilities read the Tyranid rousing from hibernation and he spies on the communication activity between each node until the hive-mind realizes and cuts all communication. Combat between Tyranid forces and the squad begins, but Nyal is kept occupied by the sheer scale of the Hive-Mind and the offer that it tempts him with; that he could join with it and be a part of the Tyranid's gestalt. Njal rejects this as he realizes he would lose his sense of self in the process. Njal is trapped and falls in a digestion chamber while the other scouts are forced to evacuate despite Sergeant Hakon's attempts to rescue Njal. The squad is forced to continue their evacuation and leave him for dead, but it is not concluded what happens.
Recently Njal, being haunted by the soul of a Thousand Son Librarian stuck in a man-made section of the Webway since the Burning of Prospero, traveled to the former home of the 15th Legion. He also rescued several Space Wolf legionnaires who were also trapped in the Webway.
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Rules wise Njal has the wolf's typical ATSKNF and acute senses plus counter-attack and independent character, and he's a ML3 psyker. He also has the unique rule "Lord of Tempests" that allows him to reroll a single failed psychic test each turn when attempting to use Tempestas powers. He can generate powers from Divination, Biomancy, Daemonology, Telekinesis, and Tempestas disciplines.
This puts him as one of the more powerful Chief Librarian psykers, lagging behind Tigurius in sheer spellcasting potential but with an improved basic profile (Captain -1 WS & I) to back him up. Comparatively, he is ahead of Ezekiel & Mephiston in spellcasting (the second has a fixed power and no one of them has casting buffs) but behind them in basic profiles too, so they balance up.
Njal goes into battle with power armor (Terminator armor for free), a bolt pistol, frag and krak grenades(loses these if he rolls in terminator armor), and a psychic hood. He also has a relic staff called the Staff of the Stormcaller that's Str+2 AP4, concussive, force, and wyrdbane (gives him adamantium will). Njal is accompanied by a cyber-eagle familiar called Nightwing that pecks out the eyes of his opponents, which makes for some epic narrative, but isn't that great overall against tough or well-armoured opponents.
His Staff of the Stormcaller is pretty impressive though. It has the base stats of a normal Force Staff, but also grants him Adamantium Will as well as the ability to re-roll one Deny the Witch attempt per turn, making the prospect of casting powers against him very unlikely. Where he can deny harnessed warp charges from ML2 casters or less on a re-rollable 3+.