Book 4 of The Tales of Alvin Maker.

In an alternate frontier where America is ruled by folk magic and mysterious charms, hexes, and potions, a man like Alvin Miller is an extraordinary person. The seventh son of a seventh son, Miller is a "Maker," with the power to create a new future for America. But first, he must defeat his ancient enemy, the "Unmaker," who threatens to destroy not just Alvin's work—but intends to destroy him. The "Unmaker" will stop at nothing until he achieves his dark goals.