There's a download PDF with the background in there.
==========COMPLETELY NOT CANON==========
This campaign was later revealed to be set immediately BEFORE Perturabo was found by the Iron Warriors. Only twelve primarchs were known at this point, 76 years into the Great Crusade.
Basically the 11th Legion were decimated in the landing on Gedren Prime, and the primarch's flagship was shot down in low orbit. It is suspected that he might have actually died, and one of the Fief Lords tried to have him cloned in secret as a peace offering to the Imperium.
A year or so later, when the fighting was all pretty much done, the primarch declared that he was taking control of Gedren and would be seceding from the Imperium. When the other legions tried to stop him, he scrambled their communications and set them all blindly attacking each other's troops - no one wanted to admit they had fired first on their brother Astartes, so a mini-war broke out on the surface with everyone suspecting everyone else of siding with the rebels.
The possibly-cloned primarch was taken back to Terra and never heard of again. Corax had his legion seize all the cloning technology (!) and that's basically it.
It was a fun storyline, and I think Graham enjoyed making a link with established canon material.