Njal Stormcaller
Njal Stormcaller, also called Njal Stormsson 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 tribesmanboy. 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.
Tabletop
Although he is able to use "regular" psychic abilities, in the fifth edition his main feature was his ability to form a driving storm that buries the enemy in snow and ice. This ability has been lost by the seventh edition, where he is now a ML3 psyker with the ability to re-roll one failed Tempestas power per turn.
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 (they both have a fixed power and no casting buffs) but behind them in basic profiles too, so they balance up.
For wargear, 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+.