This would take place at the start of the fifth year, when Harry gets attacked by the Dementors.

As we know, he uses the patronus to push the dementors back. What if he was a bit late? The idea here is that Harry suffers something of a panic attack asthe dementors approach. The dementor initiates the kiss, pulls part of Harry's soul out, and then Harry casts the Patronus. At that point the dementor slams its jaws shut and pulls back. Without letting go of the part of Harry's soul it has.

Now, I think there is a real reason for the dementors retreating from the Patronus, and that is that contact could destroy them, were the Patronus powerful enough. This dementor is point-blank, with no room to dodge. And I think we can agree that Harry's patronus is powerful. So the dementor is destroyed.

So we have a destroyed dementor, and half of Harry's soul, maybe more, maybe less, I'll leave the exact details to challenge takers, is seperate from him. This could have a variety of effects, but I am partial to it dulling his emotions, drastically.

Rules: -Harry does not become a dementor.
-Losing half a soul will not make him super-powered.
-Losing half a soul is significant. Harry's personality will change. Drastically.
-The half soul is not lost, it is merely misplaced. Possibly in his wand, possibly roaming free and making trouble.

Please, feel free to ask questions. This idea has been bouncing around in my head for months, but I lack the talent and drive and time to turn it into a story.

 

Well the first thing that comes to my mind is that beyond changing his personality by dulling it down how do you see this affecting the plot of the books. Or do you have a plot in mind that goes from here?

Edit: Just thought of this, and again this is more of a plot device than actual plot: but if Harry is able to connect with his split part he could see things in a different way, be able to affect things in a different way. You could use it to give him insight into soul magic, potentially setting it up for a plausible final battle with Voldemort. Or he could use the sight of the split soul to tamper with other people's soul, hidden fears, compulsions etc.

 

Well, this is a good way to create Harry's evil twin or start off a dark!Harry fic, but it's far from enough to call it a complete idea. It will give you material for 2 or 3 chapters, but after that, you'll need something more.

 

Since we know that emotions are not good for effective Occlumency, dulled emotions would likely improve his occlumency skills, very early on. This would probably prevent Harry from seeing any visions, thus: no warning on Mr. Weasley's poisoning, so he dies. No false vision, so Sirius does not die, as Harry never goes to the DoM. As emotions are no longer clouding his thoughts, he would also be thinking more effectively, so I suppose his grades would improve. He may even do research in the library for things similar to what happened to him. As his soul is split, it is possible that Dumbledore will inform him of the existence of Horcruxes.

 

ZOMG! I swear you stole my idea!!1

DemonDream = hacker!!!

j/k my home dawg, nice idea. (Even if you did steal my idea!)