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I generally follow the same principle as Monty with the lore books taking precedence over the novels if a conflict arises so it’s not a huge deal. There is in this lore an extended period where the Thunder Warriors and almost, if not all, of the 20 proto-Astartes first cohorts are active side by side with the Thunder Warriors (previously referred to by other authors including Alan Bligh as the Thunder Regiments but that’s a minor retcon - as far as I can see there’s always been 20 regiments - though I find the whole Astartes legions being copies a bit too neat, not to mention a bit strange given the TW are established not to be influenced by geneseed and are adult warriors with gene grafted muscle and metabolic accelerators not gene helix with personality affecting traits) The complaints from others with advance copies is of the Thunder Warriors being shown as a last vestige hunted down by the Custodians and only the 1st Legion being extant at the same time. The biggest issue, which you might be able to clarify, is about whether the author has followed the timelines established in Alan Bligh’s published Horus Heresy lore and his ‘notes to authors’ (which I have read). My comment was mostly light-hearted and based only on other posters elsewhere giving summaries. Of course, I don’t want to decry everything before I’ve read it (I haven’t). They were purely an Emperor devised varient. Though to note, she had no hand at all in the creation of the Custodes. Referenced to numerous times as their mother.
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And she went on to betray the Emperor right at the very end of Unity, destroying all the gene-seed stock on Terra for all 20 Legions, stating that without the Primarchs to stabilise the templates, the warriors would degrade over time becoming the unhinged Thunder Warriors. We know the Emperor had a guiding hand in making them all, and that Cawl (or more to say, one of his personalities) had a hand in the Astartes Project, but we have just found out that Amar Astarte led the bulk of the work, and she, besides the Emperor, was the most talented geneticist in all of human history. Not to say the least, with the creator of the Legio Catageis, Legiones Astartes and the Primarchs. There is a fair amount to change on several pages. To the editors of the page, I am happy send screenshots or list page numbers as sources, and give detailed info on the newer lore as I possess a copy of the Limited Edition Valdor: Birth of the Imperium. Hopefully these errors will be fixed in the volumes to come under the new authors. Though in my opinion, the new book, Volume 8, is nowhere near as detail-oriented and makes a number of outright canon errors compared to what Bligh did. Generally, those books circumvent the canon of the novels when there is a conflict, we go with the words of Alan Bligh and his successors. However, I would expect future volumes of Forge World's The Horus Heresy books to reference the changes. If anything in the novel directly contradicts what has previously been established it will be indicated in a relevant canon conflict section on the page, as is always done. I would also expect some changes to the Unification Wars. When the book receives its wide release, the page will be updated with as much as the relevant lore as required. The previews I have seen do not really contradict any of the existing lore they simply deepen it in a particular direction. The Valdor book is not officially published for a wide audience until February 4, 2020. It is for the fans to determine what is or is not acceptable we simply report where GW has taken the story. If it is published in an official source, it is canon. It is not up to me to determine what is or is not canon.