Redux: The Last Mage Book 6. The Last Mage, #6

After departing the town of Enterprise, Carter Nash, the Last Mage, intends upon returning to his home dimension and resuming his previous life, but he finds himself sent to another dimension and another time, a much earlier time, where he is tasked with finding and defeating a great evil, his total opposite, who intends to use the dimensional rift Carter created during his random and ill-prepared travels.
Upon returning to McLain, Mississippi, he must reclaim his role as the town's protector, giving him the power he needs. If he fails to defeat the evil, the Anti-Mage, and fails to seal the rift, the Anti-Mage will use it to merge dimensions and bring about the eradication of all existence.