so my main villan, a powerful demon spirit thing, has just been given a small group of followers by his divine Creator (me) these followers are immortal. to become a true follower you must allow your soul to absorb dark essence. i have decided that my main characters grandfather (or something) was a follower. so some of that dark essence (only a small amount) is in her. she has NO idea that its there until the end of book 3 when it 'awakens' and causes her to have rage outbursts and moments of superhuman strength. while i was jotting this wonderful idea down, i stopped dead and realised........isnt this like a horcrux? so i asked my friend about it and she said no, which made me feel better but i just want to check with you lot, because i dont want readers saying im trying to rip off harry potter :/
As long as the mc doesn't have a scar on his forehead and has ginger friend called Ron, your good to go.
Sauron's One Ring in the Lord of the Rings is essentially a horcrux. Was Rowling copying Tolkien? Your idea is far less like Rowling's than Rowling's was like Tolkien's. I wouldn't worry about it.
ideas or premises are not copyright protected... rowling got her ideas from others... every writer does at one time or another, to some extent...
Neither did I. But it did bring to mind the trailer for the "Beautiful Creatures", (I've not see the show or read the book), "The Secret Circle", and "The Devil's Advocate". All those stories are unique enough, and your description appears to have the potential to be unique as well.
So people carry this villain within them? Sounds pretty cool! I wouldn't worry about it. There are loads of stories involving containers (mobile or not) with evil souls within them.