Requiem of a Traitor (Progressive fantasy, kingdom builder)

Some burdens are too heavy for a Hero. Sometimes, they require a Monster. For nineteen lifetimes, Clay has fought the exact same war. And for nineteen lifetimes, he has watched his best friend, Stewart—the chosen Human Hero—die trying to defeat the Demon Lord. Clay is the only one burdened with the agonizing memories of their repeated failures. As the twentieth and final loop begins, he refuses to play the loyal support class anymore. To spare his best friend from certain death, Clay goes rogue. Venturing solo into the heart of enemy territory, he does the impossible: he slays the reigning Demon Lord himself. But victory comes with a bitter twist. By claiming the dark throne, Clay is crowned the new Demon Emperor, instantly becoming the very monster he sought to protect his friend from. Looking out over the devastated demon lands and its broken people, Clay realizes that humanity and demons have been fighting the wrong war. Instead of abandoning his new title, he chooses to stay. Armed with formidable kinetic Barrier Arts and a highly toxic magic sealed within his chest, Clay sets out to build a radical new kingdom of acceptance—a sanctuary where human refugees and terrifying demonic generals can stand side-by-side. But the clock is ticking. A hidden, apocalyptic threat is pulling the strings of the timeline, and Clay has just one lifetime left to forge an empire strong enough to shatter the cycle forever.