Underland

Once, the sun shone brightly on Azlant. Until came the Whitemoon, to steal mankind's light and condemn it to eternal darkness. Valdemar Verney is a sorcerer with a dream: to find a new world for his people to settle, one where the sky isn't a ceiling made of stone. But in the underground empire of Azlant, dissent is never tolerated. Brought before one of the undead Dark Lords, Valdemar is offered a choice he cannot refuse. Meanwhile, the swordswoman Marianne Reynard is given a mission: to investigate a dead cult with a few mysteries left to unravel. Both are looking for the truth, though they might regret finding it; for many cultists, monsters, and cosmic horrors stand in their way. They'll just have to kill them all. — Description from novelcodex.org