I'm tempted to do this with Fishkit.
Plot Rundown
When Fishkit is old enough he finds that there's a small voice in his head. Nobody else can hear it, it is completely separate of his own thoughts, a completely unique personality. This voice becomes his close friend, it's just a whisper in the back of his mind. He trusts this voice with his life. He finds as he grows older it starts getting louder, it isn't noticeable at first. It also slowly becomes more violent, first egging him to rip apart a living rabbit. Then escalating to acquaintances and his friends, what it says get more and more violent. He gets scared, he tries to block the voice out but it just makes it louder. To the point that he can barely hear his own thoughts, just this voice.
A medicine cat has an omen about Fishkit, they start keeping an eye on him. Noticing he's becoming more twitchy and scared. They confront him, he eventually admits whats happening. The medicine cat, feeling out their depth, decides to take him to the Moonstone to share tongues with StarClan. So they head off together, sleeping by the Moonstone Fishkit finds out that a cat from The Dark Forest is trying to groom him so they can posses his body and reign over the forest, completing their life work of murder. This cat has been doing this since he was a baby. Fishkit was just randomly selected as a baby by this evil cat.
StarClan tells Fishkit that he'll have to confront this cat and fight for his mind. At the moment this cat has 'possessed' 40% of his mind so he needs to force this cat out. StarClan explains that there's a plant that if he chews the leaves off it'll put him in a deep enough sleep to forget his survival instincts, allowing him to confront this cat. Fishkit is scared at first, he doesn't know if he wants to go through with it. Eventually he decides to do it. He confronts the cat, at first the cat tries to propose a deal. Fishkit brushes it off.
Cue epic battle sequence. Who will win?! Who will get control of Fishkit's body?!