Magic!Stiles buys syrup or something for pack movie night.
I think this was some kind of fix-it where Stiles had some OP death/necromancy powers. They were picking stuff up for movie night and Stiles or Peter bring some of the pack(probably including Derek but idk who else) to a sketchy shop that doesn't really have any identification from the outside but just turns out to be some people selling something weird like gourmet syrup or soda or something. When they leave the alpha pack ambushes them in the parking lot and Stiles drains one of their life-force.
Pretty sure this is an old fic, might have even been written pre-season 3a. I'll read almost every pairing in Teen Wolf so it could be anything from gen/slash/het; I don't remember anything specific besides this one scene. Your help is appreciated!
*** Found: The Pack, Trust and Battles by MidnightsSong https://archiveofourown.org/works/7090333 ***