The Flutes of Aeran — Michael McCloskey

Ex-soldier-turned-wanderer Keran travels into the wasteland, fleeing an unknown horror that has killed his friends. He finds safety in an isolated tower by an oasis, but the tower slowly becomes his prison, for the creature awaits him beyond its walls. During his enforced isolation, Keran slowly unravels the mysteries of his sanctuary and the monster that hunts him. Eventually all hope of resuming his previous life is lost and he becomes obsessed with seeing the creature destroyed.