This is the fourth book in what seems to be a never ending series. At first glance it seems to be a science fiction book, and it is mainly a classic space opera, but there are strong elements of fantasy. The three main races in these books have mental powers, and there are many themes of a mystical nature.
The first book in the series, "The Dark Wing," set the stage for this series. Humans were fighting with the zor, an alien bird like race which is heavy into a mystical view of the universe. The second book "The Dark Path" takes place years later when the humans and the zor have learned to get along, but are threaten by a new alien race. Some humans are dragged into the zor's approach to life. Jackie Kearny ends up on a quest for a special zor sword, the gyaryu. In the third book "The Dark Ascent" the new alien race, the vuhl, have attack the humans and zor, and initially have great success. The humans and zor learn some ways to protect themselves.
A large part of "The Dark Crusade" focuses on a small group of humans and zor, the Blazing Stars, who have learned a very effective way to fight with the vuhl. Their ultimate goal is to completely destroy the vuhl. The war changes from the humans and zor protecting themselves. It isn't even about conquest; the Blazing Stars want to exterminate the vuhl.
There are clear hints of another form of life which is trying to manipulate the humans, zor, and even the vuhl. We don't know who they are or what limitations they have. Some how this other group, life forms, or beings are tied to six colors of a rainbow. They do seem to be trying to get the humans and zor to destroy the vuhl. One thing I've wondered is are they playing a complicated chess game with humans, zor and vuhl as pieces; that the colors have picked sides and are trying to defeat the other colors. I wish the author gave us a bit more information about them.
The story is complex and rich in detail. There are many characters and the story bounces from viewpoint to viewpoint. The story moves along very well and never drags. It isn't clear that this series will have a conclusion any time soon. More and more characters, races, and threads get added with each book. The story is well written. If you like space opera and don't mind a story that leaves you hanging, then check out "Dark Crusade."