The Core / Peter V. Brett.

"A final descent into darkness" -- cover.

The war has begun…For time out of mind, bloodthirsty demons have stalked the night, culling the human race to scattered remnants dependent on half-forgotten magics to protect them. Two heroes arose-men as close as brothers, yet divided by bitter betrayal. Arlen Bales became known as the Painted Man, tattooed head-to-toe with powerful magic symbols that enable him to fight demons in hand-to-hand combat-and emerge victorious. Ahmann Jardir, armed with magically warded weapons, called himself the Deliverer, a figure prophesied to unite humanity and lead them to triumph in Sharak Ka-the final war against demonkind. But in their efforts to bring the war to the demons, Arlen and Jardir have set something in motion that may prove the end of everything they hold dear-a Swarm.