"I have very little to go back to," Severus said dryly. "So, staying here is entirely preferable. I'm sure that is a common theme. However, finding out what caused this place to be the way it is would be a good idea, if only so we know it won't affect us."
"Good. That was the point. They weren't meant to know." He grimaced. "I'd... been having doubts even before you... disappeared. But afterwards... the assumption was you'd displeased the Dark Lord for some reason and nobody would say why or how. But I didn't truly turn against him until the next year. That's when I became a spy."
"I think I've had more than enough of teaching," he said, dryly amused. "Still, taking on an apprentice or two is something they encourage you to do once you have your Mastery." He paused thoughtfully. "The quill could identify any new Muggleborn witches and wizards."