Also, love the designs on his armor resembling black vines and thorns. I can imagine him lifting up a wounded Loyalist by the throat, and the vines crawling up his arm in search of blood...
It was a full project, with accompanying Index Astartes articles and quite a bit of fluff, which can still be found quite easily.
In the end, the Space Wolves fell to Khorne, the Blood Angels fell to Nurgle (ironic, given the actual novels imply that they would've fallen to Khorne, if anyone), the White Scars fell to Slaanesh, the Raven Guard fell to Tzeentch, the Salamanders fell to Malal, and Rogal Dorn himself was the arch-traitor, with his Imperial Fists supporting Chaos Undivided (renaming themselves the Black Legion after the failure of the Heresy). The Ultramarines did not fall to Chaos, but instead became Reasonable Marines; they seceded from the Imperium alongside much of the Ultima Segmentum, and now exemplify progress while fighting off the Tau. The Iron Hands seem to have gotten mixed up with the Necrons, while the Fallen Angels won out over the loyalist Dark Angels.
On the Imperial side, the Word Bearers became the Ecclesiarchy, the Iron Warriors remained siege masters, the Sons of Horus became the crusading Black Templars, the Emperor's Children remained exemplary perfectionists (except for Fabius Bile, but he's dead now... maybe), the Thousand Sons became blind psychic warriors (see: Grey Knights), the Death Guard became the core of the Inquisition, the World Eaters became serious instead of angry, the Night Lords remained loyal Scary Marines, and the Alpha Legion were just as sneaky and just as anti-Ultramarine, but still loyal (apparently the Cabal wasn't that convincing in the Dornian Heresy).
It's pretty awesome...except that the Sisters of Battle never existed, because they were wiped out by the Word Bearers when they were still the Brides of the Emperor. Nicely done, Bolter & Chainsword!
Oh, Dorn. You just do NOT rock the traitor look.
Loyalist or traitor, he's such a dork.