The Herald of Day by Nancy Northcott

A wizard changes English history to seize power for the magically Gifted. He becomes a tyrant, exploiting and oppressing ordinary people. As his changes roll forward in time, his magical disruption of the natural order spreads famine and plague across the land.

Can an untrained seer and a cursed wizard figure out how to undo the changes? Or will the grim new reality become permanent?

Their quest to restore the true timeline takes them from the glittering court of Charles II to a shadowy realm between life and death, where they must battle one of the most powerful wizards in generations with England’s fate at stake.