Dumbledore allows bewitchers and wizardesses, wizards and witches with merpeople abilities, to come to Hogwarts. And Sirius goes. The tale of the Mauraders begins with Sirius transferring to Hogwarts. He spends time forging his path in a rapidly changing world. The Wizarding world forces Sirius come to his world from the other side as war begins.

Dumbledore allows merpeople to come to Hogwarts and Sirius voluntarily goes. The tale of the Mauraders begins with Sirius volunteering to go to school on land first and going on wild adventures. He spends time forging his path in a rapidly changing world. The Wizarding world gets intense before Sirius chooses his path as there is war again. AU Follows canon until 7th year

Dorea Black soon to be Dorea Potter nee Black, sees Sirius is neglected, and can't do anything to help him. The whole haughty Black Family and unconcerned citizens' refusal to help a child in need, puts Sirius in a dangerous situation