Brighter than the Past, Clearer than the Future

What if when Sirius fell through the veil he didn't die? What if when he fell he managed to get himself back out? But what if instead of in his time he was in the past way in the past? What if he was deaged and lost all his memories? What if he was Salazar Slytherin? What if he tore the veil? This story will answer these questions and so many more.