Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150325213156/@comment-8881468-20150418094509

Erebus is more knowledgeable about the lore of Chaos than Ahriman, however Ahriman was the strongest psyker of the Thousand Sons, master of all applications of psychic disciplines but one.. the conjuration of daemons.

If we look f.e. at the short story "the shards of Rerebus" in  the Calth anthology, i think, it is shown how much of his ability is actually only picked up by learning from his superiors and how hard it is, despite following chaos for a long while, to learn how to "cut the immaterium" open for travel. (a skill which Ol Persson, a non psyker human has mastered before), which also kinda removes that skill from being a psyker.