From the author of Pigs in the Parlor...
Two human personalities can form a relationship, and through communication and fellowship, that relationship can be enhanced. The same is true of a person and an evil spirit; a relationship can be formed, and that person can obtain knowledge or gain.
When a person forms a relationship with an evil spirit, that person then has a familiar spirit.
Familiar spirits are counterfeits of the Holy Spirit. They offer gifts similar to those offered by the kingdom of God but defiled. Anyone who is led by the Holy Spirit does not need a familiar spirit, for it is God who provides all that he needs.
As with anything occult, involvement with familiar spirits creates spiritual defilement. "Regard not them that have familiar spirits...to be defiled by them." (Lev. 19:31).