Barrenworld: Curse of the Warlord's Chalice by J. Edwards holt

Long ago, an evil wizard rose to power and waged war on mankind. Few were willing to stand against the dark wizard, and those that did, fell. Desperate and determined to save his people, a king named Astilles traveled to a realm called Barrenworld, where he found the Warlord’s Chalice, an ancient cup said to give infinite power to those who drink from it. Believing himself to have no other choice, Astilles drank from the chalice and gained the strength to defeat his enemies — but that strength came at a great cost, as the chalice only granted it in exchange for eternal servitude. Eventually, the story of Astilles faded into legend, until centuries later the chalice would return to claim its next victim.