Board Thread:Warhammer 40k General Discussion/@comment-24490059-20140203062149/@comment-69.244.7.54-20140520162228

74.128.152.190 wrote: The biggest thing that would indicate that they are NOT a mising legion is their numbers. There has been no indication that they are as big as one of the original legion or even that they have a codex-compliant 1000 astartes. Secondly, there is nothing, to my recollection, that indicates they are "exiled" from the Imperium only that they are a very clandestined (and possibly very old) chapter.

Numbers aren't the most reliable indication of status as First Founding / Legion origin. The Thousand Sons were very few in number for many reasons, but were Legiones Astartes. The Astral Claws weren't, but had more members than the codex permitted.