Geis of the Gargoyle is a fantasy novel by American writer Piers Anthony, the eighteenth book of the Xanth series.

[1] Seeking a spell that will restore the polluted river Swan Knee to a state of purity, guardian Gary Gargoyle finds himself face-to-face with the Magician Humfrey.

When Gary gets some direction, they go off to search for the philter in the middle of the madness, which is spreading farther out, in the ruins of stone hinge.

An old spell that the ancients who created stone hinge used to tame the madness, and separate Xanth from Mundania.

Surprise became way more mature after learning that she can't perform the same talent more than once, and Gary no longer has to purify the river of Swan Knee.