Source of Magic (Xanth Novels) - Piers Anthony

Ordered by King Trent to determine the source of Xanth's magic, Bink and his companions were harried by an unseen enemy determined to thwart them.

When even their protector turned against them, Bink still managed to reach his goal and carry out the King's orders. But the king did not expect Bink's next act... to destroy utterly the magic of Xanth!