The Heresy came to Calth without warning. In just a few hours of betrayal and bloodshed, the proud warriors of the XIIIth Legion – Guilliman’s own Ultramarines – were laid low by the treachery of their erstwhile brothers of the XVIIth. Now, as the planet is scoured by solar flares from the wounded Veridian star, the survivors must take the fight to the remaining Word Bearers and their foul allies, or face damnation in the gloomy arcology shelters beneath the planet’s surface. In Calth that Was, we relive the fateful encounter that will alter the destiny of Captain Ventanus and shape the face of the Ultramarines Chapter for ten thousand years to come. The battle for Calth is far from over.

Writer’s Commentary

I always assumed I’d write the war for Calth. Out of some misguided notion that because I wrote Ultramarines in 40k, I’d naturally write them in 30k. Seems like everyone else did too, as I get asked why I didn’t write the Battle of Calth quite a bit. Or at least I used to, as I’ve explained why I didn’t at more than one event. In the end it was down to nothing more than scheduling.

At one of our big Horus Heresy pow-wows at Black Library, the editors and writers realised there were several events as yet untold that absolutely had to be in a certain order to make sense of the narrative as a whole. One of those was the Battle of Calth, and the when of it was in a place I simply couldn’t do. I was neck deep in The Outcast Dead at the time, and the deadline for when the Calth book would be required was just impossible for me to meet. No matter what I did or how fast I wrote, there was no way I was realistically going to be able to write that book in time.

Step forward Sir Dan of Abnett, whose schedule did allow him to write that book. Of course I was disappointed not to get to write the Battle for Calth, but if someone else is going to do it, Calth was safe in Dan’s more than capable hands. And what a book it turned out to be! Know No Fear is a tremendous book, one that grips you by the throat and doesn’t let go until the end.

And not even then, as it turns out.

Dan had purposely left a number of elements unexplored and untouched, elements that could be fodder for future tales that he (or others) could go back to. That proved fortuitous when it was decided that Mark of Calth would be the book to pick up on those threads and weave a Horus Heresy anthology from them.

I’d already written stuff about Captain Ventanus in my Ultramarines novels, a history and mythology of the legendary ‘Saviour of Calth’, and the editors seemed to think it was a perfect synchrony to have those worlds collide. Though, to be honest, I wasn’t convinced at first, wondering how much those worlds ought to collide.

I had a sit down chat with Laurie and we thrashed out the ways the story could link 40k and 30k, ways in which it could be part of something grander and yet still stand on its own. In the end I came away convinced, and set out to plan the story. The first decision I made, since Calth that Was followed on almost directly from Know No Fear, was to continue the tense Dan had used in his novel. It took a bit of mental gear-shifting, but eventually it became second nature (to the point I’ve used that tense in a few other stories and sometimes find myself slipping into it unintentionally…).

The story became a two-hander between Ventanus and Hol Beloth (or at least that’s how it begins). I’d asked Dan and Aaron to mention a Dark Apostle called Maloq Kartho in Know No Fear and The First Heretic. Nothing major, just a namecheck or a walk-by cameo, enough to remind people of his name and presence. If you’ve read The Chapter’s Due, you’ll know that this is the true name of the daemon lord, M’kar. And what I’d written before was that it was from the ashes of Calth that Kartho rose to become a daemon…

I saw Calth that Was as a story that bridged the gap between the immediate, shattered aftermath of the Battle of Calth and the beginning of the Underworld War. The moment where the Ultramarines regroup and try to salvage what remains of their world. Where they reorganise and adapt to the new war they have to fight to win back their world. But, of course, winning back their world is impossible, the Word Bearers have all but destroyed it, and its capture was never really the objective.

The Word Bearers might see Calth as irrelevance, but the Ultramarines certainly don’t. They want to kick the Word Bearers off their world, and though it’ll never be the same again, they certainly don’t want any of Lorgar’s trash left hiding in the nooks and crannies. Ventanus was a reluctant hero in some ways, already tagged with the title of Saviour, when – to an Ultramarines way of thinking – he was just doing his job. He’d sworn his oath to kill the last Word Bearer and started by killing hundreds of them in an ambush (and the help of the orbital guns). The Ultramarines are nothing if not adaptable…

I collaborated with Dan on this story, as the tale he was penning had a nice little layer of overlap with mine. So what you read in mine makes a lot more sense when you bear his in mind. And the ending to his becomes all the clearer when you remember what happens at the end of mine.

I enjoyed writing this story, and pulling in the elements of history I’d written all those years ago – with no thought to the fact that someone would have to make sense of it all in an actual book – proved to be a real challenge. But it was a challenge I relished and making it all seem like a flawless plan we’d had in mind for years was just the icing on the cake.