The Wizarding world and the Muggle world were always meant to co-exist, from the very first days of Merlin and Arthur. There were always boundaries not to be crossed by either side, although like the aforementioned two there were always those who collaborated in tandem, symbolizing the symbiosis between the two worlds. There were those like the founders of Durmstrang, who raised their school off the Ural Mountains and hid it from all but magical eyes while on the other side there were places like Hogwarts School of Wizardry and Witchcraft where those born to Muggles were welcomed with (mostly) open arms. Its founders were four disparate individuals who crafted an improbable mix of students and staff that made it one of the most pleasant places to send children for education.
One of those Founders eventually fell out with the other three over the issue of blood purity, or so the histories said. Salazar Slytherin did not want anyone with Muggle heritage in his House and tried to extend it to the school. The histories said that Godric Gryffindor most stringently opposed this view and eventually Salazar left for parts unknown. The histories also said that he had left a giant Basilisk in a secret chamber for one of his heirs to release. When one finally came, the Basilisk was slain and it seemed that the name of Slytherin would forever collapse into evil like the monster its founder birthed deep in the caverns below.
The histories are of course, written by those that remained in Hogwarts and denied by those that were outside. Those who accepted them sought to reverse them; such was the case of Tom Marvolo Riddle, the Heir of Slytherin who felt that his ancestor was wronged by his peers and thus engaged the Wizarding World in open war and renaming himself Lord Voldemort, feeding corruption and hate throughout the globe. Yet there too were others who did not believe the words in the history books, who sought to find the truth or to live in spite of it.
This is the story of those who fought the darkness as the Chosen One and his friends ventured forth to remove the Dark Lord and his vessels of power, all in a desperate struggle to hold on to the light of truth and justice as the world darkened around them. This is the story of those who fought to keep the faith as Harry Potter became a wraith to the world in order to save it.
This is the story of the Order of Black and Silver.