A Gathering Evil — Michael A. Stackpole

Tycho Caine is a man with a mission. He's sure of that. But, waking up in a body bag with amnesia, he's not sure of much else. Except that someone wants him dead...

The time is the near future. The place is Phoenix, Arizona. The streets are roofed over with giant solar power panels called "Frozen Shade" by the street people. Giant corporate fortresses dot the landscape. A privileged few enjoy pampered luxury, while a vast under-class suffers. Behind it all, unspeakable abominations carry out a convoluted, centuries-old plan of invasion and conquest.

Caine's search for his identity and his mission uncovers an invasion from another dimension, an invasion so subtle, so covert, that most of the world has not even noticed. The invaders are the stuff of nightmares--every "mythical" monster is only a pale, dimly viewed reflection of their unimaginable reality. Worst of all, Caine discovers that he may be one of the dark minions of A GATHERING EVIL