To say that Voldemort's possession of Harry did not effect him would be a wrong assumption. In fact it took Harry from the time it happened until a week into the summer holidays at the Dursleys, for him to figure what exactly had happened to him.
Slowly Harry began to realize that he 'knew' things that he shouldn't. Like how to do spells that he had no recollection of ever seeing, let alone studying. He also knew how to do silent casting, apparating, and Occlumency. Once he came to realize what he now knew, he spent over an hour figuring out where he had learned it from. It was then that he realized a few other things. He now knew how Voldemort had 'survived' the attack all those years ago and he knew where each of the locations of the soul fragments were, including the fake one in the cave with the lake of Inferi. He also knew the means to destroy them and started with the one in his head.
A thick black mass oozed out of the scar, floated in the air for a moment before exploding into vapors. There was no sound, no scream of pain but Harry felt as if the sun had come out. Nothing could stop him now. He looked in the cracked mirror in his room and saw that the scar had faded quite a bit. Someone would have to really look hard in order to even see it. Harry grinned in realization of this fact.
The next step was to brew a potion that would remove the magical signature from himself and his wand. Luckily he had all of the ingredients on hand. It was simply a matter of timing and knowledge. Something that he now had thanks to Voldemort. The best thing about this potion was that it was permanent.
Now that the potion was taken, Harry could fulfill his 'obligation' to the Wizarding world. Then he could leave them all behind and would fade into the shadows.
During the next seven weeks Harry bounced around England, finding and destroying pieces of Voldemort. Meanwhile the Order realized that he was missing and began an all out manhunt for their missing Savior. Little did they know that Harry had paid off Vernon to sign his muggle emancipation papers, doing so after he found out that Sirius had done the same for him in the Wizarding one. Something that Dumbledore and the Weasleys planned on keeping from him. That only made him more determined to leave them all behind.
Albus Dumbledore was at his wits end. Harry had now been missing for over six weeks that they knew of. The only sign that he was even alive was a letter written to Minerva telling her that he would not return to school. Returning the Quidditch Captain badge along with it. The tracking charms on it had been removed before he had touched it. Never had Albus felt so out of control. The boy should not have been able to leave his aunt's house. He had even kept the reading of Sirius' will from him so that the boy wouldn't learn whatever Sirius had planned. It was a good thing too, given the fact that Sirius emancipated the boy. If they could only find him!
September 1st dawned bright and cheerful to those who had no idea that Harry was missing. His former class mates began to wonder where he was. They thought nothing of it even as the train pulled from the station. It wasn't the first time that Harry had missed the train back to Hogwarts.
Only Hermione Granger, best friend to Harry, seemed truly worried. She waited until the last minute to step onto the train. Harry's other best friend, Ron Weasley, wasn't so worried. He knew that Harry was missing and had been for some time, but he had gotten his one wish. The Quidditch Captain Badge. True given that his Prefect Badge was taken due to his grades was bad, but this one was more important. Something that Harry didn't have. He figured that Harry would make some grand entrance during the opening feast and all would be forgiven. He hadn't even told Hermione that Harry had disappeared all those weeks ago. When someone did ask where Harry was, Ron would reply "Training" and leave it at that.
The train ride finally ended and the students made it into the castle in record time. Hermione and Ron sat in their usual spot hoping to get a glimpse of Harry. But their friend was no where to be found.
Professor McGonagall was soon leading in the first years. For once Hermione ignored both the Sorting Hat song and the sorting itself. Her worry for Harry was too great for her to concentrate.
Just as the food appeared on the house tables, an explosion outside literally shook the school. Every Professor ran for the Entrance Hall, beating out the students who were racing for the door. Hermione was right behind her mentors and what she saw made her stop short.
Harry and Voldemort were locked in battle. A dome of transparent silver surrounded the duo. Hermione wasn't sure if it was to keep them in or protect everyone else outside. While the dome protected them, it didn't stop the sounds. Several heard Voldemort taunt Harry, but in a manner unusual for Harry, he didn't respond. Within moments several of Voldemort's followers arrived along with Aurors and Unspeakables from the Ministry. Both sides found that they couldn't enter the dome. Helpless to help the one they supported.
Hermione watched in fascination and horror as Harry was hit by several spells and returned several back to his enemy. The look of surprise on Voldemort's face when several of them did him actual damage was priceless. It was then that the Dark lord realized that he might not win this battle and tried to leave. Only he couldn't. She watched in pure awe at the amount of power that Harry was using. Not even Dumbledore felt that powerful. Everyone watch, mesmerized as Harry began to gather all of his power and sent a silent white spell towards Voldemort. He found that he couldn't block it. An unearthly scream came from the Dark Lord as the spell hit him. He then exploded into a shower of misty blood and small fragments. When that happened all those who had the Dark Mark fell to the ground unconscious.
Harry was panting heavily but still standing upright. A wave of his wand removed the dome. Hermione saw him reach into his pocket for something but he collapsed before he got it out.