Who is Remus Lupin?

Upon Alphard Black's death, Walburga was convinced her brother's fortune would be passed on to her heir and eldest son, Sirius Black. He was, after all, his favourite nephew. Yet, they are all taken by surprise when the lawyers reveal that Alphard's sole beneficiary is a boy called Remus Lupin. Just who is this Remus Lupin and why him? Magical AU.