Mutatio Skullus is a Hex. The effect is unknown, but the name of the spell suggests that it would be used to mutate the skull of an enemy.
It is one of the spells randomly shouted during the Third Task in the video game adaptation of Harry Potter and the Goblet of Fire. This may have been one of the spells cast over the Great Pyramids by the Ancient Egyptian Wizards, as Ron Weasley said that his sister Ginny wasn't allowed to go into one pyramid because inside, there were many skeletons with mutated heads.
From the Latin mutatio, meaning "I change", and an adaptation of the English noun skull.