Voldemort place (it should be "placed") a hand on his shoulder... There was a flash of red lights...

You might want to consider getting a beta because you make several small mistakes in the writing which interrupt the flow of the story. These are basic things which should be taken care of before you ever publish your story. I like your premise and you are doing a good job of realizing it. One this, Snape seemed pretty ooc. And the name thing has been overdone, calling someone by their first name to begin a relationship.