Familiars are a class of spirit - creature who are beneath the Divine and above mere mortals and dragons. Like Divines they are non-physical, being of significant power to no longer be bound by mortal form, but unlike the Divine not powerful enough to enter the physical realm at whim. Due to this, the vast majority of Familiars live within the emptiness beyond Macalgra and assorted Divine realms. Many Familiars are the lesser children of Divine, not powerful enough to immediately become gods, but destined for that fate eventually. The act point where the distinction should be made is debated frequently by mortal scholars.
A Familiar is in its original definition not a classification of a particular type of Immortal but a job description. The terminology has blurred over time to refer to any spirit capable of such a thing, and of independent mind - therefore not a Formless or Shelgoth.
Familiars have no set form, and may choose any shape when they enter the mortal realm with a mages assistance. These shapes vary from animals to mythical creatures, to humans or strange humanoids.
Familiars & mortals
Familiars primary come into contact with mortals (especially humans) when a mage seeks one out and makes a contract. As part of this contract, the Familiar is granted entrance to the physical realm by using a mortal as an anchor and may visit it at will. In return, they provide access to their magic - usually far more powerful than what a mortal is capable of.
Contracts vary significantly, and require the consent of both parties. Once signed however, they may not be broken until the death of the mortal partner. The conditions may vary significantly, and more powerful Familiars often ask for tributes or for particular deeds to done for them in addition to the right to enter the mortal realm. Upon occasion, a mortal mage may also need to appease a Familiars Divine parent before a contract will be signed.
Contracting a Familiar
The ritual of contract is used to find a Familiar. Depending on what the mage is seeking they may cast a ritual that is an open invite to any Familiar to answer, or they may be searching for a particular Familiar or the children of a particular Divine.
A open ritual is a simple matter in theory, but requires significant concentration to actually perform. Sprigs of sacred herbs are burned in a wooden bowl, and the ashes are drawn into a circle. The mage then uses any of a number of spellwords to open the ritual. Traditionally, Familiars will test a mage by waiting for a significant time before answering. As the mage must both have enough magic to keep the ritual open, as well as the concentration to do, it is used as proof of their capability if they can hold it for a significant enough time.
In order to summon a particular Divine's child or a particular Familiar, the mage must know the name of the Divine or Familiar they seek, as well as their favoured mortal tributes - usually a food they enjoyed when they were last in the mortal realm. Otherwise, the ritual is much the same.