any number of potential "Sons" of horus, there are a number of them in the legion afterall.
Though Im only listening to nemisis now so Im not up to that (have listened to vengeful spirit though)
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HAHA no, I am not.
double check which variant of the books you are looking at the release date of though, pandorax dropped pretty much with the Apocalypse 6th ed edition (little bit later as the campaign was limted ed exclusive for a bit) and Damnation only dropped last year.
On the other hand some BL authors DO post online, ADB posts significant amounts of BnC