Another detention with Umbridge and her accursed blood quill. But this time, Harry was prepared. Tonight, he would make the world a better place. He ran down his mental checklist. Wand, Invisibility Cloak, Marauders' Map, Polyjuice, knife, potions. That was everything - it was time.
Harry gathered himself, and entered Umbridge's lair. Somehow evil was all the worse when wrapped in pink cardigans and kittens. Umbridge waved a hand towards the desk, and that thin black quill.
"You know what to do."
Harry took a step towards the desk, and suddenly spun, wand in hand. A silent stunning spell caught Umbridge unawares, and she slumped in her chair. He swiftly darted forward, and plucked a single hair. Seconds later, a vial of polyjuice was ready. Surprisingly the flavour wasn't as bad as it might have been - there was a sweetness that served to somewhat mask things. A few transfigurations, and Harry had become Umbridge. He grimaced - this impersonation was possibly the least pleasant of his plans for the evening, and that was saying a lot.
Less than five minutes after Harry entered the room, he left again. But to the watchful eyes of Hogwarts' paintings, it appeared as though Umbridge was going for a stroll. They could not see the immobilised body floating beneath his father's cloak. Making use of the Marauders' Map, Harry and his captive made their way to the Room of Requirement without interuption.
The room was bare, with a sandstone floor. Setting Umbridge to one side, he began to carve the runes he would need. They were quite extensive, and some he had never tried before. Eventually they were done. Now it was time to charge them. This wasn't quite the point of no return, but it was Harry's first act of dark blood magic. With his knife he flicked a cut in Umbridge's right palm, and his own left palm. A twirl of his wand and blood was pulled forth in threads, twisting together. These threads he directed to fill the carved runes. That done, he healed the cuts, then lifted Umridge into position within the circle. He then carefully carved the required runes into her flesh, and secured her limbs in place. The following steps of the ritual had to be done without interfering magic. Umbridge would have to be awoken.
And that worried him. Umbridge needed to die - a woman who was prepared to torture a child could not be allowed to walk free in a school, and the ministry was too corrupt to prosecute her. And it was only logical to make what use he could from the deed, hence this ritual. But would he be able to stand firm in the face of her pleading? It was time to find out. "Enervate. Silencio"
"Madam Umbridge, I shall explain a few things before I unsilence you. You have been brought here to die. I cannot allow someone who is willing to torture a child to walk free. I am going to make use of you in a ritual - I need all the strength I can muster to fight against Voldemort. Unfortunately the ritual means you must be conscious, and free from magic. I hope you do not suffer too much."
With that he he cancelled the silencing spell. *This* was the point of no return. The point at which he would take a defenseless human life. It helped that Umbridge was not pleading for her life. Instead there were insults about his blood status, an admission that she had set Dementors after him, threats and bluster.
He ignored it all, and began to thrust his magic into the ritual runes. One by one they activated, the blood runes edged in brilliant glowing blue. From the runes carved on Umbridge, blood began to flow, drawn forth in twisting, spiraling cords that looped and swirled above the ritual circle. Soon, Umbridge was utterly drained, her skin a ghostly white. As the last of the blood left her, Harry began to pour his own magic into the ritual. One by one the blood runes of the circle were drawn up into the now coallescing mix. A mix that steadily shrunk in size, until just one final rune was left. Harry renewed his efforts, and with a final push that last rune evaporated. A blinding flash, and that twisted mix vanished. In its place was left a single brilliant diamond, almost perfectly clear with a faint crimson tint.
Harry Potter had created a blood diamond. Created from the condensed lifeblood of a witch, bound to him with his own blood, it would be a source and store of power that he, and he alone, could access. It was also completely illegal to possess, let alone create. It would have to be hidden carefully. But there was time to think about that later. For now, he had to clean up. The bloodless corpse he transfigured to wood, and quickly burned. Blasting spells took care of the marked sandstone. A succession of harmless summoning and hovering spells cleared his wand's memory. He then slipped on the cloak, and made his way back to Umbridge's classroom. Once there he wrote a handful of lines with the blood quill - a combination of alibi and penance. And now, the acting would begin.
Harry Potter left the classroom. Early on Umbridge had received a message of sorts, and told him to wait there. He had finally given up two hours later, too tired to care about possible further punishment. He wondered where the hateful woman had gotten to. That's the story that Snape and Dumbledore would find if they thought to look anyway.
In the end it was four days before anyone cared to wonder where Umbridge had gotten to. The minister was furious. Harry was never suspected.