There is the possiblity of loosing sanity when casting Eldrich spells.
Often used with a zombie, but can easily be used with any servant. The mage removes one of his servant’s eyes and preserves it in a black silk bag. He replaces the missing eye in his servant with a burning coal. When he wishes, the mage can retrieve the preserved eye and peer through it, seeing what the servant would see through it’s now empty socket.
This obviously affects the servant, as it will have poor depth perception. And any servant other can still see through it’s other eye, even though removed. This can affect the sanity of any living creature.