Death's Detective (Volume 1 of The Malykant Mysteries). The Malykant Mysteries, Collected, #1

To catch a killer, send a monster. There's a realm. Wreathed in ice and snow, drowning in the dark, Assevan needs a different kind of hero. There's a god. Merciless and cold, and quick to anger, the Lord of the Dead insists upon one thing only: vengeance for a murdered soul. And there's a man. A ruthless killer, Konrad is detective, judge and executioner in one. Dauntless, relentless, monstrous, he stands alone against the dark.
Meet the Malykant. Justice will be served. Four cases. Four killers. Four executions. Dark fantasy and murder mystery collide in this first collection of the Malykant Mysteries.