Biggest thing on magic used in a story is how the author has set up his or her magic system. The actual nuts and bolts behind the magic, the rules it must follow. Having these rules gives the reader the ability to suspend belief while reading and trust how the magic works even though the reader does not know all the rules regarding the magic in the story. Anyone who has played Advanced Dungeons & Dragons can relate to how those rules worked in the magic system of the game. It’s no different in a story or story universe.

In the book, “Necromancer Awakening,” and its sequel, “Necromancer Falling,” by Nat Russo, the essence or mana used to create magic comes from death and decay. If there is none or very little of the ‘essence’ left behind from death, then the necromancer is relatively powerless. What is really cool about the magic system is how it’s tied into the religion of the necromancer universe. Instead of the evil intent that most every story portrays any type of, “death magic,” the necromancers are priests that aid the souls of the departed in cleansing their souls so that they may reach their version of heaven, while healing the sick and injured and protecting the populace from those that would disrupt and harm the people of their world. While the reader, nor the characters in the story will ever understand all the rules, the rules created for the magic system in these stories are highly detailed and written in stone never to be bent or broken.

Looking at another magic system in a story that I wrote some time back that was published in an anthology, what in the story appears to all the characters, including the protagonist, as magic is in reality advanced nanite technology from their own future. In the original story the basis behind the protagonist’s magical abilities is theorized as being derived from several sources, magic, psychic abilities, and alien crossbreeding, among others. Since there is no way for the characters in the story to learn of the true source of the protagonist’s special abilities, the truth is never revealed within the story.

The thing is those nanites that create the illusion of magic have specific rules, things they can and cannot do. In this setting, think of someone from the middle ages trying to describe an M1 Garand rifle. “…A strange but heavy wooden and iron staff that when a button is pushed makes a loud noise. If you point the smaller end toward an enemy when pushing this button it will create a wound in your enemy akin to that of an arrow or crossbow bolt magically. The magic of the staff is very limited only working anywhere from one to 5 times before the magic has been used up.”

Now going back to your example of the car “Christine”
Anyone who works on or with machinery will tell you that many machines do seem to have their own personality. I’ve built and restored a lot of cars over the years, my thinking is that the builder “breathes” life into the machine during its construction. Using this as a basis for how the car in the novel became, “Possessed.” Let’s say the builder had all these negative thoughts while building the car. All those negative thoughts are passed on into the personality of the car creating the evil car.

How would this be classified in your story? That would depend on the rules regarding the magic system you use in the story. If your magic is heavily done with traditional magic of swords and sorcery. You could classify the magic as the negative energy that imbued into the car made it the perfect vessel for a demon to eventually inhabit. You could classify it as the negative energy coalescing and forming an evil entity that becomes trapped in the car. It’s all in how you want to write it, the important thing is to remain consistent in your story