Lupin had undergone purification.
Before today, Harry had not actually known what purification meant. But today, he learned.
Purification was a term used for both werewolves and vampires. After being bitten by vampires or werewolves, some ordinary people-a category that included both Muggles and wizards-would become half-human, half-vampire or half-human, half-werewolf.
Harry and his friends had never encountered the former and knew little about them. But they had one of the latter right beside them: Lupin.
As a child, Lupin had been bitten by a werewolf who had committed countless evil deeds, becoming a half-werewolf, or werewolf slave. Sirius had already killed that werewolf, but Lupin becoming a werewolf was an irreversible outcome-at least, the wizarding world currently had no way to reverse the change.
Unlike pure-blood werewolves, werewolf slaves became extremely weak after transforming and lost their reason.
Furthermore, werewolf slaves could not transform at will, only on nights with a full moon. This clearly distinguished them from werewolves, who could transform whenever they wished and were only forced to transform on full-moon nights.
Whether half-vampires or half-werewolves, however, as time passed, they would gradually become true vampires or true werewolves if they lived long enough. This process of going from half-werewolf or half-vampire to a full werewolf or vampire was called purification.
Lupin had been bitten by a werewolf and become a half-werewolf as a child, so by rights, he should have undergone purification long ago. But he had always been able to resist the process, and so, even after decades, he remained a half-werewolf, or werewolf slave.
Yet just last night, when he transformed, he discovered that he no longer became the slender monster with only a wolf's head that he had been before. Instead, he became a strong, sturdy werewolf.
Moreover, after the full moon faded