Not realy Magnus was winning that fight Russ only won cuz he got lucky.
You think Magnus, a time stopping Psyker of immense power was winning the duel to the death match against Leman, a non active Psyker ? When the duel ground to a stalemate that Leman won ?
Ahriman reached the mighty portal, and saw a battle between two brothers that was unparalleled in its savagery, power and folly. Magnus and the Wolf King struggled with the fate of a world balanced on the outcome. Forking traceries of lightning shot upwards from the ground, isolating them from the host of Wolves and Custodes.
Russ rained blow after blow on Magnus, shattering the horned breastplate, and in return Magnus struck his brother with a searing blast of cold fire that cracked his armour and set light to his braided hair.
It seemed as though the combatants had swollen to enormous proportions, like the giants they were in the myths and legends. The Wolf King's frostblade struck at Magnus, but his golden axe turned the blow aside as they spun and twisted in an epic battle beneath the madness of a blazing storm of sheet lightning and pounding thunder. This was a battle fought on every level: physical, mental and spiritual, with each primarch bending every ounce of their almost limitless power to the other's destruction.
The waters around the pyramid broke upon the shores, black as oil, and churning as though an unseen tempest boiled beneath the surface. Space Wolves and Custodes ploughed through the water, wading through the crashing spray to reach the pyramid in lieu of aiding Leman Russ in his battle. Magnus swept his hands to the side, and the warriors on the water cried out in agony as it transformed into corrosive acid, burning through ceramite plates and rendering flesh and bone to jelly.
[...]
The epicentre of this destruction was Magnus and Russ, and Ahriman watched the two titans wrestle with the bitter enmity reserved only for those who had once called each other friend. Such a contest of arms was the most desperate thing Ahriman had ever seen. He wanted to rush forward and remind them of their former kinship, but to intervene in such a planet-shaking conflict would be suicide.
Ahriman had cautioned his warriors not to wield their powers for fear of the flesh change, but Magnus showed no such restraint and battered Leman Russ with fists wreathed in fire and lightning. Russ was a primarch and such powers as could shatter armies had little effect on him save to drive him to higher fits of rage.
Magnus drove his fist into Russ' chest, the icy breastplate cracking open with a sound like planets colliding, and shards of ceramite stabbed the Wolf King's heart. In return, Russ snapped Magnus' arm back, and Ahriman heard it shatter into a thousand pieces. A blade of pure thought unsheathed from Magnus' other arm, and he drove it deep into Russ' chest through his shattered armour.
The blade burst from Russ' back and the Wolf King loosed a deafening bellow of pain. A chorus of the wolves that were not wolves added their howls to that of their master. The two enormous lupine monsters that accompanied Russ leapt upon Magnus, fastening their jaws upon his legs. Magnus slammed his fist into the black wolf's head, driving it to the ground with a strangled yelp, its skull surely shattered. With a bellow of anger, Magnus tore the white wolf from his leg with a thought and hurled it away over the heads of the milling army at Russ' back.
[...]
Ahriman looked past the approaching monsters to see Magnus and Russ locked in battle high above the causeway, the furious horror of their struggle obscured by ethereal fire and bursts of lightning. A flare of black light erupted and Russ cried out in agony. His blade lashed out blindly and struck a fateful blow against his foe's most dreaded weapon: his eye.
In an instant, the pyrotechnic cascade of light and fire was extinguished and a stunning silence swept outwards. All motion ceased, and the titans battling on the causeway were no more, each primarch now restored to his customary stature.
Ahriman cried out as he saw Magnus reel back from the Wolf King, one hand clutched to his eye as his shattered arm crackled with regenerative energies. As broken and bloodied as Leman Russ was, he was brawler enough to seize his opportunity. He barrelled into Magnus and gripped him around the waist like a wrestler, roaring as he lifted his brother's body high above his head.
All eyes turned to Russ as he brought Magnus down across his knee, and the sound of the Crimson King's back breaking tore through every warrior of the Thousand Sons' heart.P
Leman was fighting the infinitely superior combatant on paper and won so badly that Magnus' soul shattered from the loss.
.
