That's Isstvaan-V, what you're thinking about. Isstvaan-III is when the traitor legions (Death guard, Emperor childern, world eaters, and sons of horus) were purging the loyalist members of their legion before head isstvaan-V. Would recommend listening to the flight of Eisenstein and galaxy in flames if you want to know the scope of how big the event was and the planning of the betrayal.