Derek is the first to find Stiles after he realizes that all evidence points to him being the nogitsune. Matters, however, quickly get out of hand (and he gets his ass handed to him).
He goes after Stiles without any actual plan, simply hoping that the nogitsune isn’t at its full strength yet after taking control over Stiles and that he can somehow stop it. He expects to find Stiles still himself, still fighting; he doesn’t expect what he finds. It hits him hard, strong, awful stench of anger that is Stiles’, but not exactly. The fight in it is downplayed and it’s not old, like the smell on the roof of the hospital. Now the smell is strong and nauseating and Derek knows that what he will encounter won’t be Stiles.
Nogitsune seems to be expecting him, because he turns and smiles crookedly.
“I was wondering when one of you puppies is going to show up,” he, or rather it, says, in Stiles’ voice. “It could only take you so long to figure it out.”
“Sorry to disappoint,” Derek says, watching him closely. The line of Stiles’ body is rigid and alien and his eyes are lacking the usual glimmer of warmth in them.
“I feel like I should be the one apologizing. After all, I took your little toy soldier without asking,” The Not-Stiles says and smirks. “But hey, what are you gonna do? Cut your precious boy to pieces?”
Derek says and does nothing, even though it’s an open invitation. He sets out to knock whatever is inside Stiles unconscious.
“Think before you play,” Stiles says and sends him flying as soon as Derek attacks.