The TRUTH about the Horus Heresy; The Emperor was actually clearing the final proof of the 3126th ed FAQ, but Horus, irate at the wait for the update to his favourite game, invaded GW HQ to get the documents.
This move was sadly mistaken as an attack on The Emperor's sovereign status and caused the current divide in The Imperium.
The Emperor was sadly put on the golden throne after being hit by a flying model Bloodthirster, thrown by a rage quitting Angron.