A magistellus is a familiar that helps and aids the witch in her endeavors.
The magistellus may be a demonic entity who has been "bound" to some magical instrument such as a show stone or mirror, permanently or temporarily. It may also be an elemental creature formed by the combination of the witch (or wizard) power and some natural phenomenon.
The magistellus can be used both as a protective and offensive weapon. It will develop its own will over time and should be handled only by experienced wizards and witches. It is also said to be able to shapeshift into a variety of animal forms (black dog, toad, crow,...).