Telling the stories of both Etrigan and his future human host Jason Blood, side by side, Paul Cornell creates a much needed depth to the two characters. The two characters' stories bear striking similarities. Blood struggles to learn magic under Merlin and often gives in to his violent temper when he meets with challenge, and Etrigan faces defeat and humiliation at the hands of Lucifer in Hell. Both better themselves though, rising in the ranks of their respective worlds. Blood even wins the love of the enchanting Nimue (the future Madame Xanadu), and Etrigan rises to the height of chief challenger of Lucifer for reign of Hell. Dark forces conspire though and Etrigan and Blood end up becoming pawns in a bigger game...one that is emblematic of deals made in Hell.
Paul Cornell is one of the most underrated writers at DC Comics. His Saucer Country and Demon Knights are quietly establishing themselves as two of the most interesting and well written titles at DC Comics. Smart and intricately plotted, Demon Knights is more than just a medieval history of the DCnU, it's a high level dark fantasy similar to Games of Thrones and Lord of The Rings.
Artist Bernard Chang, who's been around quite some time, really does a superior job bringing Cornell's saga to brilliant life. Every single panel is packed with detail and interesting backgrounds. His grasp of anatomy and choreography are also impeccable. I've always considered him to be in the Jim Lee School of artistic style, but in some ways he surpasses Lee with his sweeping detail.
Anything that Paul Cornell is currently writing is worth reading, but Demon Knights is that rare genre crossing work that only comes along once in a while, much like a writer of Cornell's ability only does.