Magnus vs Russ according to A Thousand Sons:
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.
Magnus vs Russ according to Horus Heresy VII - Inferno:
As the two Primarchs confronted one another, the tempest that surrounded the centre of Tizca intensified - many accounts speak of sibilant whispers heard by all present, urging them to deeds they would not later speak of - and the thunderous wind tore through many of the tottering buildings and monuments that surrounded the battlefield. Some accounts, including that of Vigil Commander Krole, speak of quiet words exchanged by the two Primarchs, but all agree that the battle began with the roar of the Wolf King. (...) All about them was chaos, the frantic madness of battle and the unnatural reality-warping effect of the tempest - for as the Primarch Magnus unleashed his full power, it ripped apart the veil between dimensions and set loose those creatures that dwelt within the aether.
(...)
Here in the midst of Tizca's ruins with the aether whirling about them as a world died, its armies scattered and blood in the air, there was no creature born better suited to triumph in such a hell than the Wolf King, though Magnus would not be brought low without grievous cost. This was a contest in which he was perhaps always destined to be the victor. Despite that his brother burned him and smashed him down time and again with his arcane power, Russ rose up once more, bloody but undaunted. Minutes or hours after they had begun, for time itself had begun to fray at the fury of their battle at the centre of the aether tempest, Magnus, Russ' savage wolf-kin snapping at his heels, the Silent Sisters, by some accounts, cutting into the arch Sorcerer's control over his vaunted powers, faltered for but a moment and his brother cast him down, the final blow different to each who viewed it-for some the stroke of a blade ended it, to others the breaking of the Cyclops' back. It was a defeat played out across an infinity of existences with the same result. Leman Russ stood triumphant, wreathed in the wounds inflicted by his brother, his role as the Emperor's wrath once again fulfilled, and in its wake a storm of blinding light emanated from the sealed and embattled pyramid beyond Magnus' silent and torn corpse. When the blaze o light had faded, there was no sign to be found of any who had once occupied it, nor of the body of Magnus.
Note that this reality-warping effect that was distorting space and ******* with time is something particular to Magnus.