User blog comment:WarpSpawn/(DAT): 20 Primarchs, 18 Legions?/@comment-3258762-20111025153238

well in Canon (although i cannot remember which book) they said something like this: blah blah entire legions have been wiped from Imperial Calenders so i guess either they died or they did something so horrific the Emperor decided to remove them from existance. As for DAT, i have no idea at all :D