How did Harry know he made a mistake? Well, the searing pain in his lower forearm was a pretty good indication.
Harry quickly yanked his arm out, dragging the Sword of Gryffindor out of the dead basilisk. Blood and black venom trailed down the front half of the sword before it was seemingly absorbed by the metal. Harry, however, could not afford to look at this peculiar sight as he felt the pain start to travel up his arm.
Quickly Harry looked up to assess the current situation. Tom was taken out by Fawkes who burned atop Riddle’s diary after taking a curse from the owner. Harry knew Fawkes couldn’t give him tears so soon after burning. He had read up on phoenixes after meeting Fawkes in Dumbledore’s office.
With that information observed and absorbed in record time thanks to adrenaline, Harry made a choice. Harry switched the sword to his other hand.
He settled the blade over his elbow before placing it higher past mid bicep, better safe than sorry. Before he could lose his nerve he brought the sword down.
Harry cried out around clenched teeth as he fell to his knees. The sword had only made it halfway, stopping in the bone. The angle and his own body fighting against him, deprived his swing of any power. He pulled the sword out of his arm, a sickening sound reverberating as he dislodged it from the bone.
Taking a deep breath in he brought the blade down once more, making it through the bone, but still failing to make it to the other side. His arm clung to his body by a thin piece of skin and tissue, desperate not to let go. The pain was almost worse than the Basilisk bite. The bite had been burning whereas this pain was sharp. The sensation of not being able to feel the lower part of his arm was completely alien.
[2:54 AM] With a final roar to the world above, Harry brought the sword down, finally releasing his arm from his body. Still the job was not finished. He dropped the sword after using it to stand as he grasped the stump in an effort to stem the blood flowing from it. The blade drank his blood, thirsty for more, as it returned to a pristine shine.
Harry stumbled over to the nearest torch on the wall leaving a trail of blood. He could feel it flowing down his arm and soaking his robes. When he reached the wall he fell against it using it for support as his breathing became more labored. He unhooked the torch and then pressed the metal guard against his stump cauterizing the wound. The nauseating smell of burning flesh warred with the iron like scent of blood.
It was almost funny, the burning of the torch felt eerily similar to the burning of the venom. All he wanted to do was fall asleep, but he knew if he did that he might not wake up. Slowly he got to his feet a bone-deep tiredness filling him as the adrenaline seeped from his system.
As he staggered to the exit, he saw his reflection in the water along the side. His hair was a mess; his face and clothes covered in blood and grime; and he was missing an arm, his dominant one at that. Now how was he supposed to do all of the chores at Privet Drive. It didn’t matter he was alive.
He had survived.