Impossibility, Impracticability, or frustration of purpose
Way to discharge performance.
Impossibility - performance cannot be done. Must be objectively impossible, cause by unforseen event that was unavoidable and could not have been guarded in K.
Impracticability - performace become extremely, unduely, or unreasonably difficult, expensive, or burdensome.
Frustration of Purpose - central purpose of K has been under-minded