Flashback:
Snape was waiting in his room after he had heard that Dumbledore had left. Hopefully everything was all right. Last time Dumbledore went on a crusade it had cost him his hand. He had been shocked when he had found Dumbledore in the hospital wing where his hand was slowly being consumed by a curse, a curse he had invented. Immediately he knew there was no cure, he could stop the curse consuming Dumbledore whole body but he could not heal the wound. He had to see how one of his own curses hurting the one he loved.
Suddenly he heard a commotion outside his rooms and a rushed rapping on his door
"Severus, there are Death eaters everywhere and some students are hurt. We need to do something!" He heard squeaking behind behind the door.
Great Flitwick. The little professor had always been nice to him. Any moment the death eaters roaming through the castle could discover him and when they did he would have to kill him!
He quickly opened the door and there was Flitwick with behind him The bushy one of the terrible trio and Luna if he remembered correctly.
Fliwick quickly came in and closed the door behind him
"Stupefy!" Severus whispered as the small professor just turned around. It was the only thing he could think of at the moment and it wasn't a minute to late. He picked up the little man and laid him onto the couch making sure he was alright. As he walked to the door he remembered the girls. He quickly opened the door and told thm thate Flitwick passed out and pushed them inside.
He had just started to walk towards the hall when two death eaters came around the corner Crab and Goyle senior judging by their posture.
"Come on the fight is in the entrance hall" He heard the plump voice of Crab
He had no choice then to follow them into the entrance hall. The sight that greeted him there was terrible. Students were trying to hold of death eaters that were sending curses to them without pausing. Then he saw what he feared most. He saw Draco Malfoy rushing up the stairs through a barrier that apparently only let death eaters trough. And he remembered the unbreakable vow he made with Narcissa. This must be the task she was talking about that Voldemort gave Draco. He fought his way to the stairs and when he finally reached it he quickly ran after him. It must have taken him a good few minutes and when he reached the tower door and ran through it he knew that getting death eaters inside Hogwarts wasn't Draco's only task.