Fenrir Greyback is a werewolf who is involved with the Death Eaters but does not rank highly enough to carry the Dark Mark. He is known as the most savage werewolf ever to live and is greatly feared throughout the wizarding world, even by his natural allies. He positions himself close to his victims when the moon is almost full. To fulfil his agenda of creating as many werewolves as possible, Greyback has infected scores of people including the young Remus Lupin, future teacher to Harry Potter and member of the Order of the Phoenix, when Lupin's father had angered Greyback. Greyback is known for targeting young children. Unlike most werewolves, Greyback thirsts for blood even in his human form. Fenrir is the name of a powerful wolf in Norse mythology.
In Harry Potter and the Half-Blood Prince, Greyback first appears on the night of Albus Dumbledore's death, when he attacks Harry and badly scars Bill Weasley. Although Greyback does not transmit his lycantrophy because he was in human form at the time, Bill is subsequently shown to display a preference for very rare meat.
In Harry Potter and the Deathly Hallows, Greyback leads a Snatcher gang - Dark wizards looking for Muggle-borns and "Undesirables" in exchange for gold. When Harry accidentally uses Voldemort's name after it has been made taboo, Greyback is alerted and his gang attacks their camp. Harry, Ron, and Hermione are kidnapped by the Snatchers and are taken to Malfoy Manor. Bellatrix Lestrange promises Hermione to Greyback in return for his services, but the prisoners fight their way out and escape, Greyback being hit by a triple Stunning Spell. Greyback is the main werewolf within the pack that agree to aid Voldemort in the Battle of Hogwarts. During the battle, Hermione, using a blasting curse, prevents Greyback from attacking injured Lavender Brown, and a crystal ball thrown by Professor Trelawney stuns him. He rejoins the battle in time for the Death Eaters' last stand, when Ron Weasley and Neville Longbottom combine forces to take him down by magic.