Well, this was an unexpected surprise.
Released at a considerably cheaper price and less fanfare than the average Supplement Codex, even lacking that exact name despite the exact same format and type of book, Legion of the Damned is surprisingly good. While hardly outstanding in terms of rules, the lore aspects of the book seem to have been written by someone who understands what made them work. Answering the mystery and removing all ambiguity surrounding the force would have destroyed a lot of interest in them. As such, much like Cypher - Lord of the Fallen, it seeks to flesh out the mystery with more ideas and fact. Not destroying or utterly scrapping what we had before, or or trying to write the codex as a novel like so many books, but instead using and augmenting it.
The book itself does not start with any specific beginning outlining the origins of the Legion of the Damned, but covers theories and ideas surrounding them. In a somewhat similar manner to the original Codex: Necrons, it covers outsider's perspectives on the force. Theories from the Inquisition, efforts to understand their kind and even the documented theories of heretics suggesting their possible goals and origins. Nothing is written in stone and what is made only draws in greater interest, allowing fans to draw their own conclusions far more than many others books would allow them.