Complex sentence: Fenrir Greyback is a werewolf who is involved with the Death Eaters, he works alongside Lord Voldemort because Lord Voldemort promises fairness to werewolves around the country.
Split into simpler sentences: Fenrir Greyback is a werewolf who is involved with the Death Eaters. He works alongside Lord Voldemort because Lord Voldemort promises fairness to werewolves around the country.