Anathema - Heart of Darkness, #1

"In days of old, spirits came from another world. They joined with man to guard this world. And so began the Uiscogeni."Hidden in the shadows, the Uiscogeni are guardians of the Earth, watching the millennia roll by as humanity evolves. Aldric doesn't realize it, but his father's decision to reinitiate contact with humans after two thousand years is going to change his existence. Uiscogeni lifespans make them essentially immortal, so any real impact on his life should be minimal.
Until a random encounter with an evil spirit nearly kills him. But it isn't one of his own people who rescues him. It's a human. A young woman named Helen he can't get out of his head. Something about her calls to him. Before he can decide what that something is, a terrified stranger crashes into their lives, bringing with him a danger greater than any the Uiscogeni have ever faced. One that threatens not just Aldric, Helen, and all those they hold dear, but the entire world.
The Uiscogeni soon find themselves staring into the abyss, facing the most terrifying evil the universe has to offer. With no other choice but war, Aldric only knows one truth. Whatever the outcome, more than one world will be shaken to its core.