This is the Amazon summary:
Once the legendary knights of the Gabala defended the nine duchies. [...] But they were gone, disappeared through a demon-haunted gateway between worlds. Only one held back [..]. Now he was the coward knight, and in torment.
Murder and black magic beset the land. Rumors circulated that the king was enchanted, changed, that his soul was dead . . . and that a reign of terror was about to begin. Now Manannan realized he would have to face his darkest fears: he had no choice but to ride through that dreaded gate and seek out his vanished companions.
To mention a few ideas I particularly liked:
- How the idea of "colors of magic" are treated. This is an evocative, coherent magic system. I could see it influencing Magic: The Gathering (and certainly my games). Also, it has a very interesting take on magic items, which require not only special efforts but also special materials.
- One of the best portrayals of a bard that I've read in fantasy, mostly because he has an important role to play instead of being a comic relief.
- How the "gateways to hell" are treated, somewhat reminiscent of The Elder Scrolls IV: Oblivion. The whole "hell dimension" part is really interesting.
- Monsters that appear out of their natural habitat, creating a contrast between fantasy and realism.