There are a host of 40k content creators that specialize in lore. Luetin09 is usually my go to. He just released a 40k beginners guide, but it's nearly 2 hours. There were 20 Primarch created by the Emperor genetically engineered creations his sons and each was given their own legion. Two legions were erased for one reason or another writers left it open. The other 18 were split by the Horus Heresy. 9 of the legions turned traitor and became corrupted by the Chaos gods. So the antagonist is a member if one of the traitor legions.