One of the first things I was taught when creating a magic system was that there always had to be a cost, to keep the magic interesting. After all, magic cannot solve every problem is the cost is just too high. Now this could range from mana point, human sacrifice, the shorterning of one's lifespan, etc.
So, considering this, I'd add a high cost as the reason why not every witch keeps a familiar around. Maybe the familiar is stealing the life essence of the witch and shortening her life everytime she uses its power. Or perhaps the familiar's presence and the power needed to keep it anchored to her world is draining her of her humanity and driving her insane.
At the end, I'd say you need a drawback to explain why not everyone has got a familiar and why she isn't the most powerful witch ever by having one.
As for the whole creepiness factor, I'd suggest keeping the scenes minimalistic. Show, don't tell... but don't show everything. Show just enough to unnerve the reader and let their own imagination do the heavy lifting.
That's not to say you should just have her say she did "unspeakable things" and be done with it. But you could try, for example, explaining what she felt instead of showing what she did and let the readers fill in the blanks.
I saw a show awhile back in which an evil priestess called the red woman gave birth to a shadow demon meant to kill her enemies, it seemed like an interesting concept to try, so I want to include it in my story. In this world, a witch has the power to summon a familiar. These demons are spirits summoned from an alternate realm called the aether and exist in servitude to their master. They are forever linked with their master, and used in a number of ways, such as magical batteries and amplifiers, or for battle. They grow in power with the user, changing their form and growing larger with age.
This form of dark magic involves the witch using herself as a conduit between the spiritual and physical realm for the spirit in an ancient ritual, filled with chanting and subsequent materials. During the ceremony, the spirit enters the womb of the individual and gestates for about one month. After this period of time, the familiar is birthed into the world.
I want to do this from the perspective of the individual. Due to the strong visualization, I would appreciate any tips or advice on how to write it. There is supposed to be an element of creepiness to it, but not seem tacky or cheap. What should I show or gloss over? How much is too much? Another problem I am having is thinking up a reason why most witches don't create familiars. I was hoping people could give me some ideas on this.