Shortly after the deaths of the Founders, the Flame Freezing Charm is created. The Wizarding World having enough of Muggles forcing them into hiding, decide to use the Charm's creation to fight back. The Wizards win and as a result, the International Statute of Secrecy isn't created, the world hasn't moved on from the Middle Ages, and Hogwarts School of Witchcraft and Wizardry has been abandoned in favour of splitting the four Houses into four Kingdoms, with each descendant of the Founder ruling as the King or Queen respectively. Despite their rocky history, the Kingdoms of Ravenclaw, Hufflepuff, and Gryffindor have maintained the values of their Founders by holding a peaceful attitude towards Muggles and Muggleborns.
However, not all is well for the Muggles and Wizards alike. The Slytherin Kingdom has also kept the beliefs of their respective Founder and turned Muggles and Muggleborns into slaves. To make things worse, the Gaunts have been murdered by the illegitimate heir of Slytherin, Voldemort. Voldemort has crowned himself King of Slytherin and waged war against the other three Kingdoms in order to rule all the four and become an Emperor.
Meanwhile, Severine Snape, a young maiden burns with the desire to prove herself as a great witch. She had been bullied by both her peers and her magic-hating father, and her best friend had married one of her childhood tormentors. Believing that there is nothing for her in Gryffindor, she sets her sight on joining Voldemort's Knights of Walpurgis. The best friend, Queen Lily of Gryffindor, knows that should her friend ever dare to join Voldemort, Severine would either be killed on the battlefield or executed for treason after her trial if she was captured. Worried about Severine, Queen Lily begs her husband's friend, Sirius Black, a former heir of a Noble family in Slytherin, to help her ensure Severine does not leave the land of Gryffindor.
She never expected Severine and Sirius to fall in love, though.