Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150325213156/@comment-6078851-20150417142451

i thought the ban was on psykers in general - astartes librarians were allowed to remain but they had severe restrictions placed upon them (as well as imperial psykers), and the librariums were to be phased out... weren't they? the relaxation on psykers came after emps' carcass-ification but the only psykers that were to be allowed according to emps' strictures were navigators and astropaths etc (and emps was a psyker but unlike any psyker before or since, and remember that he firmly believed and practiced "do as i say, not as i do" :P). i'll have to go and read the council of nikkea again :)

as for ahriman being a chaos space marine, i suppose that's technically true, but the thousand sons in general, or at the very least ahriman, despise the forces of chaos and tzeentch in particular for manipulating them and their primarch, despite the fact that their continual quest for knowledge fuels tzeentch. ahriman was actually banished for performing the rubric which prevented chaotic corruption, as such his psychic powers are all-natural and his physiology, and the rest of the thousand sons who survived the rubric, is still astartes. so, granted he's a chaos space marine, but the thousand sons have their own bracket - no longer loyal, but neither chaos worshipping nor chaos corrupted astartes, but still 100% astartes. i'm not trying to find loopholes in your posts but i think ahriman is a loophole :)

i don't know too much on pre-heresy erebus besides that he was a chaplain. if he was a psyker then surely he would've been a librarian? (but as you say we can't know 100%) as for the fight that would be tough one i think, the best natural sorcerer against a powerful chaos-sorcerer. i'd probably bet on ahriman wiping the floor with erebus though - i'm not taking anything away from erebus but ahriman is unmatched in psychic skill.