Harry becomes an adult but due to a Mail Wards someone else takes his power

Another Harry Potter one. Due to his name coming out of the Goblet of Fire Harry is now recognized as an adult by the Magical World. The only thing is to fully gain control of his family he has to claim his family rings which are being kept by the Goblins of Gringotts within a certain amount of time or lose it. The Goblins send Harry a letter to let him know about this but due to a Mail Ward which Dumbledore has on his that prevents most letters from coming to Harry the letter is redirected to someone Harry knows giving them a chance to take full control of Harry's family lines, vaults and due to having control of his family magic even affect him to a certain degree,