The Descent into Madness

The Descent into Madness by Tessa Rae and Sharon Cross is a Highlander novel of the highest caliber. Set in an alternate universe in which Duncan and Methos meet while Tessa is still alive, the novel adds more elements into the Highlander universe, including a shapeshifting mutant Immortal created by a secret, sinister branch of the Watchers. Macleod feels an instant connection with Adam Pierson, even though he doesn't quite trust this Immortal posing as a Watcher. But when Adam's fiancee is murdered by James Horton, Macleod brings Adam home with him. From that point forward, the two have to deal with the murdering mutant, fanatical Hunter Watchers, and bad news Immortals out to take the head of the eldest Immortal. Go, read. (Umm, you might want to block out some time, though. Once you start, it's hard to stop and the story is novel length.)