Dude, you're good at this.It depends on Calth, I think. There are four possible authors (Kyme and McNeill for Ultramarines; Reynolds and ADB for Word Bearers.) in my eyes, though the pairing which LotN mentioned will probably write it. The reason I don't think it is the Word Bearers, is due to the release of The First Heretic being soon. I imagine that we will see Signus and another novel before then, but I don't think this is actually the start of the Calth duo. (Which is obvious, in my opinion..)
What DA said.