Hollow King by Dante O. Greene
It’s been five years since Barridur Sharrow deserted the army rather than commit war crimes. He now works as a mason, he’s well-liked by his community where no one knows his past, and he focuses his time on raising his adopted child. But everything falls apart when his former commanding officer, the Grand Mage, returns to the city and discovers him — and shortly after that, Barridur is executed as a deserter.
Barridur finds himself in Hell where he meets the fabled Hollow King. A cruel and capricious god, the Hollow King offers Barridur a chance to return alive to the living world.
All Barridur has to do is defeat the Nine Champions of Hell. An immortal minotaur, a shapeshifting crow with a flock of man-eating ravens, a banshee who lives in a well, a flesh-eating swarm of flies, a cannibalistic chef, an enchanted stone giantess, an unstoppable rock troll, a huge two-headed hound who guards the Gates, and a final unknown entity no one has lived to tell about. How hard can it be, really? All he has to do is beat them all and survive. If he fails, the Hollow King will devour his soul and Barridur will become part of the wraith army, enslaved for all eternity.