Harbinger: The Downfall - Book 1 by Travis I Sivart
The Talisman has graced the sky as expected. The comet has visited once every hundred years since time began - but this time, it didn't leave.
In a land torn by dark conspiracies, a power hungry Duke topples kingdoms for his own amusement and hidden purposes; a Dasism crushes cities in a war to recover stolen lands; the magical emanations of the celestial visitor increases the powers of an insane necromancer, and attracts terrors from the bowels of the earth. The chaos lures others from below who seek to wrest control of the land, as an ancient species rises from the subterranean tunnels with their demon army.
The strange events throw together five people from different walks of life with the knowledge that the world as they know it is ending. Ripped from their lives, their actions will decide the fate of the world.