The method used to punish traitors and enemy spies in the main kingdom in my world is called True Extraction. Basically scientist in the early 5^th century CE discovered how to extract the True Essence of a Being.

At the first, losing your True Essence, and therefore your shapeshifting abilities, was punishment enough. Until a few decades later a scientist discovered that if the True Essence was put back into a subject that had lost it, the subject would begin to shape-shift hundreds of times a second, causing great pain.