Board Thread:Warhammer 40k General Discussion/@comment-24490059-20140203062149/@comment-74.128.152.190-20140203224343

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.