Originally Posted by Child-of-the-Emperor
The events of Pandorax are also covered in Codex: Black Legion:
"...Before the portal closed, however, Abaddon took with him a psyker, rumoured to be of prodigious strength and the bearer of a unique gift that would aid the Warmaster in his forthcoming Black Crusade."
"From Fulgrim, Primarch of the Emperor's Children, the Despoiler was gifted with the favour of Slaanesh in exchange for the Pythosian psyker, offered as an unblemished vessel to contain the avatar of Fulgrim's god."
It's not exactly conclusive, but if Epimetheus was the psyker that is being referred to then it opens up some interesting avenues.
"The theme behind Warhammer 40,000 isn't that the Imperium will one day emerge triumphant. It's the chronicle of how humanity ends, inch by inch."
When does this book take place?