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Snape appears at the burrow on Harry 16 birthday badly injured
Molly tried to heal him but no avail
Severus tried to tell them to use his own healing charm. He asked to see Dumbledore when he got better
Severus told Dumbledore about young man (same age as Severus when he became deatheater) who wanted to be a cool spy but knew nothing about being spy). Voldemort knew he was a traitor and brought his two years old daughter and let his deatheaters took a turn to torture the poor child. At last Voldemort turned to Severus and ordered him to torture her to prove he didn't go soft after associated with Dumbledore. Severus looked at her pleading eyes and killed her. Voldemort got angry that Severus killed his toy so Severus become his toy instead
