A/N: First Story so please don't be too harsh on me. I own nothing, only the little plot bunny that hopped into my head. Read and Review people!
SPOILERS: HP2 and HP7
Everyone knows that Hogwarts was founded over a thousand years ago by the four greatest witches and wizards of the age; Mario Gryffindor, Peach Hufflepuff, Daisy Ravenclaw and Luigi Slytherin.
For a few years the Founders worked in harmony, seeking out youngsters who showed signs of magical talent to be educated at the castle. But disagreements sprang up between the four. Slytherin wanted to be more selective about the students admitted to Hogwarts, he believed that magical learning should be kept in all magic families. Unable to persuade the others he decided to leave the school.
Rumour has it that Luigi Slytherin built a hidden chamber that the other founders knew nothing about and before departing ,sealed it until that time that his own true heir arrived at the school. The Heir alone could open the chamber and unleash the horror within and purge the school of who, in slytherin's few were unworthy of studying magic.