Tempus Unbound — Janet Morris

Is this the Lemuria of antiquity, or of times to come? Once you've ridden the storm clouds of heaven from the edge of time, anything is possible.
Demonic hordes threaten to destroy the very fabric of time itself. The fate of all humanity rests on the shoulders of Tempus the Black, Favorite of the Storm God. But even this hero of legend will encounter a challenge he has never faced before . . . present-day New York City.