BTVS episode "Wild at Heart", not sure which book Greyback shows up in.
Not mine, just playing with Joss and JKR’s toys.
Oz was sitting in a cell at the Ministry of Magic when Willow and the others came to get him. They’d had one hell of time getting that far. The ministry was convinced that Oz was a dangerous killer and they weren’t about to let him go. By the time they had everything straightened out Willow had gotten the story from Oz. Seems he’d killed a man while in his wolf form. Willow was totally shocked, that just wasn’t Oz.
The man he had killed was one of the worst the closed world of wand magic had to offer. Not only was he a werewolf that reveled in turning others, maiming and killing, he was also one of Voldemorts allies. That’s why the ministry was so afraid of Oz. Fenrir Greyback had been a hunted killer who had eluded capture for more than 20 years. For someone as small and mild looking as Daniel Osbourne to have killed Greyback, well he had to be something more than “just” a werewolf. To convince Willow, all Oz had to say was, “He was Veruca’s father”. Then it had all made sense.