It's situations like this that highlight the very real problem with having someone as powerful and cutthroat as Severus Snape off anyone's leash without another ten years of life experience to temper him. He realizes, almost before the spell's run its full course, what a very bad idea this is, but it is very much too late. His own arm burns and hurts so badly at the proximity to the epicenter of the spell that he recoils and staggers back into the wall. It means he misses being immediately charged head-on by a barely-corporeal manticore, at least.

The immediate force he has to use to banish the Patronus necessitates that he drop the charm hiding him, and Sebastian is met with the sight of one twenty-five-year-old Severus Snape against the stone wall across from him, left forearm being held defensively by his right hand, expression furious and disoriented at once.

But there's only so many things that can cause that sort of reaction. His gaze turns searching, a note of something like panicked horror in his face- It can't be, can it?