Creating a horcrux in something that can think for itself is unlikely to end well. The being can think and act for itself, and is therefore capable of 'damaging its physical body beyond repair', or to be more specific, coming in contact with potent magics, which are quite likely to kill them.
However, creating a horcrux within a living being does have its advantages. It will form a type of telepathic bond between the person and their horcrux, a bond only matched by that of the a soul bond (such as marriage soul bonds, phoenix soul bonds, and basilisk soul bonds). This is likely because all four are a variant of a bond that heavily involves the soul.
Given time (as well as displays of kindness and other 'pure' actions of the heart), the soul can heal. This is possible even if a part of the soul has been turned into a horcrux, although such a case is much harder. If the witch or wizard truly feels remorse for what they have done, they can potentially make their soul whole again. Doing so, however, has a high chance of killing them, and they are therefore unlikely to attempt it.
It is worth noting that one must intend to create a horcrux for one to be made. If they do not (but still kill with and for pleasure), they will merely split their soul, without gaining the benefits and problems that come with the creation of a horcrux.
The necessary steps to create a horcrux are quite simple, yet equally putrid. One must first kill someone, and do so for the sake of pleasure, amusement, or something similar. Once you have killed, you must think of the memory before proceeding by - '
Harry put the book down, not even remotely interested in learning more on the disgusting creation of a horcrux.
"You were right." said Harry, "I barely understood the first half dozen paragraphs. But those are likely the most horrible things I've ever heard of."
"Ah, so you must have stopped at the point where the book got to its creation?" asked Salazar.
Harry, however, was not at all listening. He was beginning to question why Salazar was showing him this, and the only logical conclusion was one he dearly hoped was incorrect.
"Did Riddle make one?"
"He planned on creating seven."
Harry could not contain himself. He bent over the side of the desk, puking out the remains of his lunch. He continued to cough for a few moments, before he vanished what he had regurgitated. When he finally stood up, it was with very weak knees.
"He's immortal." said Harry weakly, "He shredded his soul."
"Indeed he is, and indeed he has. But tell me, what have you observed within the text you read? Anything you might recognize, or find familiar?" asked Salazar, his voice uncharacteristically soft and caring.
"No, I didn't see any - "
Telepathic connection. Think and act for itself. Contact with potent magics.
"I believe," said Salazar softly and slowly, "That you are a horcrux."
Silence.
Harry had no external reaction - it was something he had seen coming moments before the portrait had said it. Internally, however, he was horrified. How much of himself was, well, himself? How could he possibly survive this war if Voldemort possessed immortality for as long as he lived?
Drunk on emotion, Harry's eyes began to slowly water, regardless of him ruthlessly attempting to suppress any and all emotion.
Harry was not crying to throw a tantrum, nor because Voldemort was quite clearly immortal. He was mourning the life he could have had, free of the control of others. A life where he would not have to do what others had planned for him to do, but one where he chose what happened to him. To live long enough to finally discover his king piece, whatever it was that he was fighting for, whatever it was that mattered to him above all else.
"How?" Harry croaked out, "How can I possibly fight him? How can I be free?"
"There is a way for you to remove the horcrux within you, without dying." revealed Salazar, "But it will be immensely difficult, and is entirely theoretical."
"All I need is possible." said Harry weakly.
"You must work on your Occlumency to an incredible degree. Once you have succeeded, you must allow yourself to be hit with the Killing Curse once more, preferably by Voldemort himself."
I could have sworn I said possible.
"He's half dead." said Harry, "He doesn't even have a physical form."
"He will eventually, I am sure." said Salazar, "Before I banished him, he was working on a ritual that would allow him to return to a physical state if needed. He only needed a few critical components, and more importantly, someone to get them for him."
"Pettigrew got away." muttered Harry angrily, "And Trelawney told me, she said, 'The Dark Lord will rise again with his servant's aid, greater and more terrible than ever he was.' Sirius was right . . . we should have killed him."
"What you could or should have done is of little importance, it is now set in stone and can not be reversed." said Salazar, "What you must be more concerned about is how to react in the future, so that you might be victorious."
Harry sighed dejectedly, slumping to the floor.
"Just tell me everything I need to know."
"I believed that your mother used a sort of Blood Ritual to give you protection from the Killing Curse. By dying to save you, she could have potentially powered a ritual that would render you immune to the Killing Curse, but I am not sure how long the protection would last. There would be much more in the complexities, but let us leave it at that."
"So she's the reason I survived?" asked Harry, suddenly finding himself becoming more depressed.
"Yes, I do believe so." said Salazar, "It would have explained why you were chosen for the horcrux rather than something else, perhaps an object or nearby muggle. You are a living being, and one with a powerfully ritualistic form of protection. The horcrux will have seen you as the best potential choice for a host."
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