The Unfamiliar, A Rather Nasty Magical Parasite...

...which can travel from sphere to sphere clinging to a Spherewalker's body. They know when someone is looking for a familiar. They oblige and find a place for themselves. Inside or outside the Spherewalker. 

The Unfamiliar finds a suitable place to cling to a Spherewalker and be carried to another sphere.

Mature Unfamiliars look like a suitable if unpleasant-looking familiar or companion for a Spherewalker. The Unfamiliar uses Illusion Magic to create the impression that they are augmenting their host's powers and abilities. Gradually, they sap the life energy and vigor of a Spherewalker.

Eventually, the Spherewalker host will become so weak that s/he cannot finish an astral journey from one sphere to another. Reaching this point takes a number of spherewalks equal to the Spherewalker's Earth score. 

When a Spherewalker collapses between the spheres, this is when the Unfamiliar reproduces. It releases a swarm cloud of embryos: barely noticeable gaseous apparitions that lurk in the astral substance between the spheres.

Pity the innocent Spherewalker who breathes-in one of these swarm cloud embryos on their journey between the spheres. The Spherewalker will carry the infection into the next sphere they visit, whereupon their physical form gradually begins to warp and shift with each successive spherewalk. 

They will begin to take-on the appearance of an unpleasant or diseased creature. After a number of  spherewalks equal to their Earth score, the Spherewalker experiences a precipitous decline, falling completely victim to the parasite. Shrinking, shifting, the infected Spherewalker takes on the semblance of a potential familiar and becomes a mature Unfamiliar. The cycle of infection continues.

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Mature Unfamiliar:

Fire 1, Earth 2, Air 5, Water 6; Illusion Magic: One Element, Magic Level 6