The Fey are a ruthless, bloodthirsty, magic-wielding, conquering people. The people of Blue Isle, the next on the list of places to add to the Fey Empire, are peaceful, religious, and have never even encountered magic. The Fey expect to grind the Isle into the ground in a day. Instead, through luck or coincidence, the Islanders discover that the holy water used in their religious rituals kills Fey on contact, in a particularly gruesome fashion. The battle shifts, and the Fey end up fleeing, going into hiding in an invisible fort in the middle of the Island.
What follows are years of skirmishes between the Islanders and the Fey, who are led by the son and granddaughter of their Black King, Rugar and Jewel. The Islanders' king, Alexander, is forced to change his entire political outlook from a peacetime footing to winning a war, while his son, Prince Nicholas, finds out that war isn't as glorious as he thought. Meanwhile, the religious leaders responsible for making the holy water that is now used as a weapon have to reconcile their peaceful religion with the horrific things their water does to the Fey.
This is one of those books where no one comes through unscathed, and sometimes the characters you like the best are the bad guys. The writing leaves something to be desired sometimes, and Rusch has a tendency to repeat herself in an effort to pound certain concepts in, but overall, an enjoyable book with an ending that makes you anxious to hear more.
(Important Note: While books 1,2,3, and 5 are still available, book 4, The Resistance, is out of print and generally unavailable. I read the series without it and was able to follow it fairly well, but if you're the type who absolutely cannot read a series out of order, don't even start The Fey.)