Expels Horus' aura

As the Warmaster swung up the maul, Russ twisted free of the claw, shredding his own armour and flesh to force an opening, and with every ounce of his strength he thrust the spear one-handed into Horus' side. A shockwave blasted from the impact, Russ' face he pushed on, grinding the spear through the outer layer of Horus' Terminator plate, into the armoured undersuit, through the body glove and into his brother's flesh. Horus looked down at the weapon protruding from his flank in disbelief. A thin sheet of blood ran down the glistening black ceramite of his plate.
' I do not need to win,' said Russ.
Howling, Leman Russ pushed again, plunging the eager tongue of his blade into the Warmaster's guts. Horus roared in agony, and his men faltered in dismay. His maul fell from his grip and he began to shake tremendously. His head jerked back and a blast of white-hot soul fire blazed from his mouth, cracking the armoured cowl curved above his head. Skittering lightning crackled over the two brothers. Violet light blazed from his wound, and the edge of the blade shone golden. It too was shaking, it edges blurring, becoming a spear made of nothing but light. Russ' arm shook painfully. His post-human muscles and bones went numb as he struggled to hold the weapon in place.
Still screaming light, Horus staggered back, releasing Russ in his attempt to dislodge the blade. Russ would not relent, and went with him, grinding the weapon in the wound. The Warmaster gripped the edge of the Emperor's Spear, desperate to keep it from cutting deeper. The scream ended, the white-hot light of his wounded soul cut out, and he fell to his knees, head bowed.
When Horus looked up, the unholy aura had gone from around his head. The absolute confidence he had displayed a few moments before was absent. His flesh hung slackly upon his skull. He had aged a thousand years in a moment.

Wolfsbane