It is introduced with the Sons of Nagarythe
warband (Shadow Warriors), Mordheim Annual page 61.
The warband was first published in Town Cryer 10. The familiar can be found on page 27.
We have been using familiars ever since and never deemed them unfair. There are not many powerful spells and especially in the beginning a wizard has some stupid spell he successfully casts every third turn or so. I don't think that wizards are overpoered and this item sure isn't either.
By the way, you might like to have a look at the newly introduced Academic skills for wizards in the Annual (page 87). Wizards that may cast two spells per turn or re-roll whatever is needed - one or both dice -, this is where it's getting a little ugly. Not that