A magistellus is a familiar that helps and aids the witch in her endeavors.
Origin
This 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.
Powers
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.