Another ex-GW staffer who now writes for Black Library. Guy also ran British sci-fi magazine SFX for a while. After a promising start with books like Skarsnik, Champions of Mars and Crash, Guy has mostly gone for middling Black Library pulp. Because you love that shit. He is also one of the few writers in Black Library to write stories from the perspective of Orks (and DEYZ IS GRaTFUL FOR DA PUBLISITEE).
More recently, Guy has established himself with forays into the Blood Angels, Dark Imperium and the Horus Heresy. All of which prove several things.
He's awesome at Blood Angels. He made Dante so cool not even Mary Sue Syndrome can touch him. He gave the Blood Angels fortress monastery its badass name "Arx Angelicum". He detailed their narrow victory over Hive Fleet Leviathan following the disaster at Cryptus.
He has a knack for writing Primarchs, both the Loyalist and Traitor ones. He also has been scheduled for the largest number of Primarch books so far, including Perturabo, Corvus Corax and Konrad Curze. Currently he's taken up the role of turning Guilliman into an awesome (ish) protagonist with the Dark Imperium series.
He's also disgustingly descriptive. In the Dark Imperium: Plague War, he writes several paragraphs worth of a man being turned (literally) inside out by the powers of Nurgle and it's almost as horrible as the Daemonculaba.
Not Ian Watson.
He's apparently one of the fastest Black Library authors.
He also looks like the love child of Roose Bolton and Martin Freeman.