The foe was terrible in its strength, and even the wiles of veteran Astartes did not stop one of their own from falling to one of those two-handed blades, leaving a terrible wound and a dead battle-brother as the foe sought another victim. 

The intercession of the others helped turn that grim tide; and the fight started to move the other way. All the while, the sounds in his ears and the altered vision, the product of unusual lights and things that flashed around, as vision itself shifted subtly and unpredictably, making one question their balance and, more fundamentally, their sanity. It made the fight with these things even more desperate, as something gnawed at them and laughed all the while, promising them torment, pleasure, power...

He'd heard the seductive likes before and perhaps it was his lot to be eternally tempted. The Primarch had fallen, and perhaps they shared the flaws that made them extra attractive to the things seething out there. Perhaps it was easier to prey on their emotions, knowing they were cast adrift lacking their genetic founder.

Even that doubt was deadly, and caused him to falter a moment, realize his error and come back at the red-skined demon with a roar and a renewed ferocity. It wasn't unthinking fury, but the thought that he was faltering and abandoning his brothers in the fight that carried him through; a chainsword gripped in both hands and brought down into the skull of the thing that he was duelling with, the blades ripping through flesh without remorse, roaring down through horn and bone as he pushed it harder and harder into the thing. The blade might well be ruined in the fight, for chainswords were not invincible, but at least he'd kill the beast. And if it didn't do the job completely, he'd find another way. 

He wasn't going to be taken, not this time...