"Potter," she began, turning to face him. His arms instinctively wrapped around her waist and he buried himself against her stomach, body shaking. She lowered her voice, speaking to him in a manner she'd heard people use on their dogs. "Harry. You did very well. I know it is scary, but you did wonderfully with the spell. It only took you two tries. I am very impressed. To celebrate, would you like ice cream for lunch?"
"Ice cream?" Harry whispered. "I've never had ice cream before!" He couldn't hide his shy, pleased smile. He was good at magic, and he had earned ice cream.
"Then we will get ice cream," The necromancer said, trying to keep her voice softer as she patted the child on the head. She was not well equipped to deal with crying. Or children. Or feelings. "But first, Harry, we have to finish up our work. Your uncle is still alive, and we have more to complete. It is cruel to let him linger in this state. When things become overwhelming, it is best to get the task done quickly. Save your panic for afterward." The boy nodded, wiping his tears away.
Harry sat down again and the horcrux was removed from his head. It truly was a phenomenal piece of spellwork, and had it been encased in anyone else, she would have studied it gleefully. The shard of Voldemort struggled against her spell, having firmly rooted itself in the scar, so it took some painful tugging at the horcrux for it to finally hiss and fly out of him. With a swift blast of fiendfyre the screaming soul piece died.
"That is another spell you are not to try on your own," she said, eyes narrowing at the very curious face of her apprentice. "You'll burn down all of London if you aren't careful. Now, I know that was painful, how do you feel?"
"Weird, different." His hand cupped the bleeding scar. "I hadn't noticed anything about it before but it feels better. Will the scar go away now?"
"No, but it should fade considerably. The horcrux was keeping it from healing, but a cursed scar like that will never go away." She crossed the room to her shelves and pulled out a box of heavily enchanted items. "Pick something out of this box. I would prefer a piece of jewelry, but if it's an object we can keep it here under the wards."
As Harry began to carefully dig through the pile of objects, the necromancer continued her lecture.
"Now, a horcrux. A horcrux splits your soul into two pieces. It's calculated that the split takes out between fourteen and eighteen percent of the soul. As you are young, I expect that this split will leave you feeling considerably detached, as you haven't reached maturity in body, mind, or magic. I think in this case it is important to set aside a specific part of your soul so that you don't end up accidentally giving it all your emotions or memories. That would make you quite insane.
Now for the construction of a horcrux - the ritual takes three steps. The first is the most complicated - the splitting itself. This requires a sacrifice and the correct mindset - the soul does not naturally wish to do this, obviously. As your uncle is still alive, I believe he is best suited. You must willingly, and without deep remorse, kill him with the curse avada kedavra. Next, is the incantation which will separate -"
"K-kill him?"
"Yes." She very, very politely tried to hide her annoyance. Honestly, it was only her luck to be stuck with such a crybaby, moral apprentice. "You have already basically done so, Harry. I had not expected him to survive the Spirit Drain. He will pass on soon - nothing can heal him now. If you had been a few years older and suffered a while longer he would have died instantaneously. You were willing for that, this step is no different."
"I didn't know that spell would kill him!"
"I did warn you there would be some violence involved in being my apprentice. I did tell you murder is not off the table. I have not deceived you in any way. You signed the parchmentwork and took the oath. You cannot back out now." She stepped closer to him, her eyes hard and glowing brightly. Her hands were clasped behind her back and she regarded him with a look of coldness he had not even seen on his relatives. Harry looked away in panic and sniffled loudly. The necromancer sighed, forcing in her temper. She reminded herself that this was a literal child - a six-year-old boy and if he had been frothing at the mouth to commit murder that would have been very bad for other reasons. Harry was not yet accustomed to the morals of necromantic magic. She would train that out of him, but he needed time. She continued in a flat tone. "We are using your relative as a benefit to you. He is already functionally dead. First by my actions and now yours. My first kill was a complete stranger - an innocent muggle. Would you rather kill an innocent or the man who physically abused you for several years?"
Harry started to cry again, his stomach twisted into sharp knots and he no longer wanted ice cream. While everything she'd said was true, it did not make the situation any less overwhelming.
"I don't want to look at him," he begged after a long moment. His mind was whirling. He wanted to learn magic, he needed to learn magic, and this was the cost. It was fine! It was fine, he didn't have to look and he could pretend it wasn't happening to a real person. It was pretend, like a game or a dream.