So, I am the Seeker for Puddlemere United in Season Four of The Quidditch League Fanfiction Competition and this is our Practise Round. Originally Round 7 of Season 2.
Prompt: Harry's scar.
Word count without A/N: 1,032
It's not well known, but horcruxes do influence the behaviour of those around them. Think of how everyone in the Golden Trio became moody while wearing the locket or how depressed Ginny got while writing in that diary.
But those instances only lasted for some weeks or months. There is one documented case of someone living with a horcrux for over a decade.
The Dursleys.
Before taking in his nephew in, Vernon Dursley was not exactly a nice person, sure. But that just made the work of the horcrux easier. Being near a horcrux brings out the worst in you. It brings out your worst fears and does its best to make you a horrible person.
The horcrux took Vernon's xenophobia, his fear of the unknown and his arrogance and blew them to previously unknown levels.
The result was that Vernon thought his way of life was the only acceptable one. Had Harry not been a horcrux, he might not have started stealing from the company worked for. He might not have started cheating on his wife. Might not have started neglecting and abusing Harry (which was indeed counterproductive for the horcrux).
The horcrux took Petunia's jealously, her desire for a normal life and her pride.
The result was that Petunia kept grudges over literally everything; even for something as simple as someone who arrived later than her getting served before her. Had Harry not been a horcrux, she might not have insisted on that over the top housekeeping Harry was forced to do. She might not have spoiled Dudley to this extent. Might have seen the truth and told Harry about his parents and maybe even Severus Snape.
The horcrux did not have much to take from Dudley, he was only a toddler after all. So it simply ensured that all the bad characteristics he would develop stayed, while the good ones were reduced.
The result was that Dudley quickly stopped playing with his cousin and started hurting him instead. That he started to take everything from sweet little Harry. That he started to think he was the king of the world, the center of the universe and everyone should treat him as such.
They recovered when Harry was away, sure, but it simply wasn't long enough for them to return to their previous selves.
But they weren't the only ones unknowingly in close proximity of a horcrux for several years.
The horcrux increased Ron's jealousy and laziness, Hermione's desire to get everything under her control, even Percy's selfishness. Seamus lost his ability to think for himself, instead becoming a mindless follower. Fudge got more self serving (not that he wasn't before). It made sure Severus's grude would never go away (which again was counterproductive).
Harry himself fell victim to the horcrux too.
Before Harry Potter got his famous scar, he was a bright, friendly and energetic child. Always giggling and halfway to being a Marauder already.
After he got it, he became different.
He nearly never laughed. He didn't show his intelligence. Didn't call attention to himself unless absolutely necessary (even though the Dursleys had something to do with these characteristics too).
Because with Harry Potter the goal was to stay hidden, to survive, not to be seen as a neutral or good thing if found out (even though that rarely worked).
Thus Harry Potter did his work quietly, be it chores or homework, while never showing his true potential with the latter one.
The horcrux made Harry like cooking, it did need him to survive after all.
It also controlled Harry's magic.
Limited it, so it rarely showed during his childhood. And then only after weighing the consequences carefully. With the obvious exception of healing injuries, the vessel was of no use of the horcrux if he was dead, worse, it would die too.
Even in Hogwarts, the horcrux made sure Harry didn't seem powerful enough to get many enemies (just enemies, not mindless sheeple who followed the masses) with just enough "accidents" to be seen as powerful, but too lazy to do serious harm.
The horcrux influences his curiosity, removing it from school subjects not vital for survival, instead putting it to mysteries.
It tried to prevent Harry from destroying other pieces of its parent soul, but sometimes it was needed for its survival. In the Chamber it was either kill or be killed, as the older piece didn't recognize its younger counterpart. It simply had no way to.
The soul piece wanted to be together with the others, which was why Harry wrote in the diary, why he forgot to take the locket off. The locket, not knowing the fact they were of the same soul, tries to destroy the vessel in an attempt of revenge. Once the younger got the older one's attention it would have been too late, had Ronald not arrived.
Both of them tried their best to stop the two full souls from destroying the older of the two, but it was pointless. Full souls were more powerful in their situation.
Nagini - the only one younger than the one in Harry's body, the only one who was able to recognize it - would never have tried to kill Harry had it not been for original-soul-piece's orders.
Which made second-youngest despise original-soul-piece.
So when original-soul-piece tried to kill second-youngest for the seventh or eighth (perhaps even ninth) time, second-youngest let it, but not before doing its best to take original-soul-piece down with it.
The others were all gone, after all.
Wasn't it pointless to hang around when original-soul-piece did not listen to it, didn't acknowledge its existence and even actively tried to kill it?
So it did not try to stop Harry from going to meet original-soul-piece. Didn't try to stay in its vessel, but instead tried to drag original-soul-piece out of his.
It nearly worked too.
So when its vessel and his headmaster talked in limbo, he could not be helped, because he did not want help.
It had lived separately for nearly seventeen years and while that was not the longest time, it was still fairly long for a horcrux, and it saw more of the world than any before it.