Board Thread:Warhammer 40k General Discussion/@comment-4851119-20140118095944/@comment-6078851-20140209155936

Vaughn891 wrote: Well if the Emperor ordered him to do it he must have done something profoundly bad, like very very heresy level bad.

not necessarily - it may have been that the primarch in question had a very specific duty to perform. maybe this primarch wasn't of the same calibre as the rest of his brethren, physically and mentally, and after performing the single duty that emps had for him it may have been that emps saw no need to keep him (or her) around, and like the thunder warriors he removed what he felt was no longer of use - emps has a record of being utterly disdainful of that which is of no use and a primarch would be no exception.

of course it could be that it was something heresy level bad, but then that doesn't explain why the records were expunged. none of the traitor legions have been expunged and you would think that, at the very least, horus would have been the most viable candidate to be removed from imperial records.