You're pretty lucky your friend is into horror stuff, or else this type of costume wouldn't fly. The clerk comes out of nowhere, and asks you, "Need any assistance?"
You're kind of confused, but regardless you mention something about looking for the fitting rooms. After all, what use is a costume that doens't fit you?
The clerk smiles, with a very austere grin, saying, "Oh, the fitting rooms are in back. You might need a bit of help with this one..."
"Oh, no, I'm fine," you say.
"No, I insist. This costume's a hassle if you don't know how to... wear it."
The glint of this guy's smile is really starting to freak you out. But, the hopeless person inside of you doesn't listen, and walks off towards the fitting rooms.
While thinking about that really strange guy, you pull the costume's main piece over your head. Ironically, it fits you quite well. It must have been sheer luck that you managed to find the perfect size. As you're trying on this costume, you wonder if you friend's dad, the special effects guy, might hook you up with some fake blood, just to pull a really good prank on his birthday. And then, your thoughts slowly progress as you wonder, what if it was real blood?
These thoughts increase, until you realize you're slowing dreaming of attacking people at the party, drinking them dry...
But you're not a vampire... Are you?
You finally realize you've finished putting on the costume (is it a costume?) and look up in the mirror.
What you see doesn't look anything like the old you, but it sure doesn't look like a costume. You feel the fangs (did you put in the fangs?) in your mouth, and start to gaze more...
Your skin is paler than a ghost, and your eyes have lost their color to dark blackness. Your hair, once short and human, is now long, gangly, black. You have grown a few inches, too. And that costume makes you look older, by at least a few years. But you know, it's more like decades...
What's happening to you?