God of Broken Things — Cameron Johnston

An outcast magician must risk his body and mind to save the world from horrifying demons in this “kick-in-the-nuts, edgy, and dark” epic fantasy adventure (New York Journal of Books)

Tyrant magus Edrin Walker destroyed the monster sent by the Skallgrim, but not before it laid waste to Setharis, and infested their magical elite with mind-controlling parasites. Edrin's own Gift to seize the minds of others was cracked by the strain of battle, and he barely survives the interrogation of a captured magus.
 
There's no time for recovery though: a Skallgrim army is marching on the mountain passes of the Clanhold. Edrin and a coterie of villains race to stop them, but the mountains are filled with gods, daemons, magic, and his hideous past. Walker must stop at nothing to win, even if that means losing his mind. Or worse.