The foundation of love and loss

Salazar Slytherin is often named the villain. History has written him as the course of the fall of the founders, and has called him one of the darkest wizards of all time. It is true, that Salazar left Hogwarts after a fight with Godric Gryffindor, but not willingly and not alive.
What happened between the four that used to be the best of friends?

This is the story of how friendship leads to deceit and love leads to murder.