Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150717204414/@comment-25177957-20150912060008

It might be true that only a being with as much psychic power as the Emperor can defeat Abaddon, having defeated a Horus clone means a primarch probably couldn't best him, not without some help anyway. Someone such as Eldrad with help might have been able to finish Abaddon off.

If Ahriman learns the secrets of the Black Library, which he may never learn, he could certain kill Abaddon, or atleast incapacitate for an extended amount time.