Garth Nix is a perfect name for a dark fantasy writer. It follows the same style of his prose, slightly archaic, but also mysterious and fantastical.
The coolest concept explored by Garth Nix is death. In his Abhorsen Chronicles books, death is, literally, a river. When someone dies, they enter the river and float through the nine precincts of death. In between each precinct is a gate. The sections of the river vary over its course, each more creative and interesting as the last.
When you're writing about death in a fantasy series you must have...necromancers! Not only does Garth Nix include mages of death, there are good ones and bad ones. Obviously, the bad necromancers are antagonists, while the main protagonists are good. The leader of the good necromancers is the Abhorsen, as featured in the series' name.
This review is somewhat incomplete, as I haven't read any of Garth Nix's writing beyond the Abhorsen Chronicles. It'll be a while before I read anything else of his because of other books, but I strongly recommend checking out everything he's written. The Keys to the Kingdom is one of Mr. Nix's better known series. His YA space opera novel A Confusion of Princes will most likely be the next of his books that I will read. That is, if he doesn't finally finish Clariel...