Dan Abnett in the official author commentary for The End And The Death: Volume three

The duel, I felt, should propagate the same way. It begins conventionally enough (albeit at a spectacular power level), but then accelerates. This is where that 'extra gear' was critical. The clash also happens across several 'rounds', instead of one long bout like Sanguinius' duel. The first of those rounds – the longest single chapter in the novel, I think – is told from Horus' point of view, and demonstrates that gear-changing escalation. It is breathless and unremitting. The 'conventional' fighting shifts to psychic war, revealing that it has been waged on multiple levels all along. To achieve the unfolding of this first shift, I used locations in the way I had previously used weapons (so, 'He parries me with his sword' becomes 'He parries me with Cthonia'). We understand this is still a fight, because of the sentence structure, but the weapons have changed, and thus the manner of the fight must have seamlessly changed too. The fight is expanding outwards, and proliferating. It is everywhere. It is physical and in the mind. It is operating on multiple levels. Then we shift again, and extend the fight to more magical areas, employing aspects and the infinite dimensions of the warp.
My intent, which I hope has been realised, is that each time you think the fight can't get any bigger, it does. The final gear shift (of the first bout at least) is into the use of Tarot. As with location, tarot cards – or, more specifically, the value and meaning of tarot cards – become weapons, wielded and deployed by sentence structure in the way that swords and claws, and then places, were previously wielded.
Their fight, like the novel itself, constantly presented me with the same key problem: if we're starting big, how much bigger can we go? At what point will I simply run out of superlatives that are bigger than the ones I have used before? There were times when I genuinely wondered where else I could go to make it (as the bard has it) 'one louder'.

-The End And The Death: Volume three