‘It was the biggest armoured engagement in Imperial history.’

‘It lasted for close to a year.’

‘It was estimated that there were ten million tanks and war machines involved, and by the end there were a million wrecks on the surface.’

Now, the largest armoured engagement in human history so far is thought to have been between about six thousand tanks and four thousand aircraft. If you add those figures together on the basis that they are both types of war machine, it gives 0.1 per cent of the forces apparently involved in the fighting on Tallarn.

So it’s a pretty big battle. The #TenMillionTanks! fact gets that scale over really directly. It’s a staggering figure, and one that has always stuck with me.

But why do I sometimes wish I had kept that oh-so-juicy phrase behind my teeth? Because it sets up an expectation that the stories I was going to write would be all about seas of tanks shooting the hell out of each other. And that was not at all what I intended to write.

Having got to this afterword you probably know that, while there is a lot of tank-killing-tank action, the touted #TenMillionTanks! do not take centre stage. And that’s because what drives the stories in this book are five basic questions:

How did the Battle of Tallarn start?

What was it like for those fighting the war on the ground?

Why did it become so big?

Why did the Iron Warriors come to Tallarn?

Why did the battle end?

That in itself was a fairly daunting list of points to address, given that the conflict was so huge and lasted for so long. In the end I decided that the battle just wouldn’t suit a conventional single story structure – what was going on was too big, and had too many strands to put into a traditional Horus Heresy novel. Like the wider series, I wanted a feeling of there being more stories than could ever be told, and that there were truths and secrets that even those who found themselves at the heart of the action would never learn. One of the concepts that my editor, Laurie Goulding, and I used when talking about the possible structure of this book was the Battle of the Atlantic during World War II. There have been separate novels and films set in that conflict that focus, for example, on allied destroyers escorting convoys, others that are just about U-boat crews, and others still that are set a long way from the fighting, focusing on the role of code-breakers and spies. Or, to put it another way, stories about what it was like at the sharp end on both sides, and stories about all of the secrets that drove the strategic flow of the conflict. It’s one narrative, across many stories.

The solution that I settled upon was not to write one story that tried to address everything, but several that would each focus closely on one perspective of what was going on.

‘Executioner’ is the story that shows the experience of the rag-tag human tank crews facing the Iron Warriors immediately after the invasion. There could be different stories about any number of other tank crews, but by focusing very closely on Tahirah’s squadron, I wanted to try and convey not just the action, but the very human aspect to the battle, facing off against legionaries purely because they have to. ‘Siren’ goes back and gives the answer to why the battle blooms out to an even larger scale after the Iron Warriors meet this slightly unexpected resistance on the surface.

‘Ironclad’ looks at why the Iron Warriors came to Tallarn in the first place, and why they later quit the field, leaving the loyalists victorious…

…and ‘Witness’ is a brief footnote about the price of that loyalist ‘victory’.

The stories are all largely unconnected by characters (that three-headed son of the Hydra, Jalen, not withstanding), but was does connect them is that they are all stories of individual heroism, betrayal and tragedy.