This description is from Wikipedia:
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.