The Reflection Crack'd (The Horus Heresy)

The Emperor's Children have thrown their lot in with Horus, but their own path is taking them into the clutches of the Chaos God Slaanesh. While the warriors of the Legion descend into debauchery, Fulgrim's closest allies notice that their primarch has changed, and plot to purge the Phoenician of the daemonic influence which is clouding his judgement.Written by Graham McNeill. This story is also available in The Primarchs, Book XX of the Horus Heresy series. Read it because
It showcases the lengths the Emperor's Children will go to in order to save Fulgrim from daemonic corruption, and the surprise twist will make you rethink your views of the primarch...