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human
[40K] So, Horrus stabs the Emperor, the Emperor kills him.. What are the events immediately proceeding this?

He's standing there in a battlefield bleeding and what not I guess, Horrus is dead, so his men just sorta... stop fighting? So who suddenly took charge and got him back to the throne (which I assume was nowhere near the battlefield, being at the very bottom of a palace the size of Europe...) Who started setting up contingency plans in the case that at that moment he *died*.
gpt
They're on board Horus's battleship, the Emperor has just wiped Horus's soul from existence and is lying, nearly dead, in the midst of horrible carnage. The bodies of Ollanius Pius and Sanguinus lie near him, and the Emperor weeps for instant of sanity he saw in his son's eyes before he killed him.

Rogal Dorn enters, having split away from the Emperor shortly after they teleported aboard the battleship. The Emperor, realizing that he is dying, and knowing that he needs to be alive to guide the Imperium, tells Dorn how to build the Golden Throne, and gives him instructions on how to keep the Emperor in stasis. Dorn then teleports himself and the Emperor back down to the surface of Terra, where the Heretics have been pushed back in the wake of their General's death. 

Abaddon retook Horus's battleship and lead a fragment of his forces away, where they would eventually hide in the eye of terror. Other Primarchs took their forces and fled as well, some of them being tracked down and killed by Imperial forces, others managing to escape.