The writing is not the high point of the videogames. Neither is the characterization. Or the worldbuilding. To be blunt, these games are about great visuals and really excellent level and puzzle design. I was curious if those qualities could be translated into prose. Surprise! They can't. It's not a very good novel.
The setting, if you're unfamiliar with the games, is an extended Miltonian universe -- demons, angels, Hell, Heaven, and a liberal mash-in of whatever else the designers could think of. The protagonists are the Four Horsemen, here retconned as the four survivors of the extinct Nephilim. They go around pissing off Heaven and Hell on the orders of the mysterious Charred Council. The games take place after an Apocalypse-gone-wrong destroys the Earth. (Yes, Apocalypses can go wrong.) The book, in contrast, is set long before the birth of the human race. A cache of superweapons turns up; Death and War are sent out to deal with it before either the "good guys" or the "bad guys" can make a mess.
The author makes a serious attempt to capture the high-speed button-mashing combat of the game. Lots of lovingly-described acrobatic butchery. There's also lots of lovingly-described scenery (some beautiful, more horrific, just like in the games). It recalled the games to mind very well, but I don't know if non-players would get anything out of it.
The same goes for the characters. War and Death (plus their siblings^ Fury and Strife, who would have starred in the next two games) are basically grunting hulk-monsters waving oversized anime weaponry. They get to be sarcastic in cut scenes, is the extent of them. To his credit, the book author sets up some character development -- the Horsemen have an actual sibling-like relationship, which is informed by their already-eons-old history as immortals and survivors of a dead race. But there's not a lot to it.
(^ Fury is a Horsewoman, in case you were wondering.)