The Castle in the Sky - A Sword & Sorcery LitRPG Dark Fantasy with Gods, a War, and an RPG-Combat-Style Final Battle Between Good and Evil. The Golden Wand Trilogy, #3

The quest of the Golden Wand seems doomed when Zandrew learns that Ankezebaur the Last Avatar of Light is dead, but a slim glimmer of hope still remains, and Zandrew and his friends cling to that slender hope and venture onto one final quest, to try to piece together fragments of the Great Secret, which will enable them to use the Golden Wand to defeat the forces of darkness before time runs out and the armies of the Dark God Vladius take total control.
But once Zandrew learns what the Great Secret really is, his faith shatters, and he achieves the power to save the world but at a terrible price beyond what he ever imagined.