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Albus Dumbledore knew many things. Who wouldn't after nearly 115 years of life. He new 15 uses for dragons blood - 3 were dangerous and Dark so he chose not to add those in his published work on the subject. He new many of the secrets of Hogwarts, secrets that only the previous headmaster had been privy to. He knew much about the workings of Magic, Destiny and Fate. He knew that Magic just was. It existed much like light, or energy, with no ability of its own but it could be manipulated by others; he knew that Destiny itself could not be changed, but that many paths led to ones destiny and it was up to the individual to chose which path to travel; and he knew that Fate was Destiny's guardian, making sure that all the roads laid before a person truly led to their destiny.
Albus Dumbledore knew a lot that other people did not know. These were his personal secrets, not book knowledge, logic or deduction. Only he knew he has an aversion to all things coffee. Only he knew the real reasons that he chose to dedicate his life to Hogwarts instead of climbing the ladder at the Ministry of Magic. Only he knew the extent of his relationship with Gellert Grindelwald. Only he knew of the broken time turner that dangled around his neck, under his eccentric robes. It was the same time turner that Harry Potter and Hermione Granger had used to help free Sirius Black. The very same time turner that Harry Potter had broken when he trashed the headmaster's office the night that Sirius Black had fallen through the veil at the Department of Mysteries. Since that night the old wizard had taken to wearing the time turner to remind himself that his mistakes could not easily be fixed. Not even Albus Dumbledore himself knew how many times in the last year he wished that the time turner had not been broken.
Albus Dumbledore knew he had made many mistakes in his life, some repeatedly. Early in his teaching career he came across an orphan, Tom Riddle. Having no children of his own, Albus always had a soft spot for children who were lacking family. He, as well as the rest of the Hogwarts staff, would turn a blind eye to some of the trouble Riddle would get up to. Because Riddle was an impoverished orphan, they did not think it was kind to punish the boy or treat him harshly. When Tom Riddle turned himself into The Dark Lord Voldemort, Dumbledore promised himself he would not give into the soft-spot that he had for orphans again. This being the case, when Severus Snape entered Hogwarts Dumbledore would not coddle him. Oh yes, he loved the boy genius, whose mother had died 2 years prior to his entering Hogwarts - and whose father had all but abandoned him at around the same time. He loved the child but saw right away the boy's interest in The Dark. Dumbledore refused to treat him with kid gloved, if anything he held young Severus to higher standard than his year-mates. This of course backfired spectacularly when Severus Snape took the dark mark directly out of school. So when Harry Potter, the most famous of wizard orphans, had entered the school the headmaster refused to take action at all. In addition to leaving Harry's discipline to others, he also ignored the boy's complaints about his treatment both at school and at home with his muggle family.
Albus Dumbledore knew he was dying. Not because of his age, as a powerful wizard this age was not unheard of. No, Dumbledore knew he was dying because in a moment of weakness he put on a heavily cursed ring. The ring was one of Lord Voldemort's horcruxes - the items imbued with pieces of his soul that would help keep him bound to this realm. When Dumbledore had found the ring in the old house that had belonged to Voldemort's grandfather, he saw something in the ring that he thought could give him one of his hearts desires. It was a foolish mistake by a foolish old man. Dumbledore being who he was tried to make the best of the situation. He knew of Voldemort's plans to have him murdered and planned accordingly. The headmaster asked his spy, Severus Snape, to commit the murder instead of letting fall to the hands of a student.
"Severus... Please...", he pleaded. He was not pleading for his life, but for this most trusted and loyal follower to carry through with their plans.
Albus Dumbledore knew regret. He saw the pained emotions in Severus Snape's eyes. Just over Snape's shoulder, Dumbledore could see the spot that Harry Potter stood, frozen and silenced under his invisibility cloak. Regret washed over the aged wizard, so deep that he felt overwhelmed by it. Regret that he could not help his two favorite students more. Regret that these two had grown up lonely, neglected and misunderstood. Regret that they had been treated so unfairly - that he had treated them unfairly, that he had failed them. Regret that so much had fallen on their young shoulders in a war. If only he could make their lives better. If only... At that very moment, Severus Snape cast the killing curse, hitting Albus Dumbledore squarely in the chest - precisely where the hidden time turner lay.
Albus Dumbledore knew there was nothing else he could do. The greatest wizard that ever lived, closed his eyes as he felt himself falling and spinning away.