It wasn't a costume that you dropped now that you have a good look at it. Although, it may have looked like one before it suddenly transformed mid-fall. You picked up the key, and it looked like one of those old cartoonish-looking keys. The one with a circular end where the key chain would be, and the other end would be two cylindrical notches. You look back at the assistant to ask for the key. Then you saw it. A brief unnatural grin that reached from ear to ear. Her smile died quickly but you know that she knows about how unnerved you were by it. You tried hard to erase the memory from your head.
"Congrats!" The petite assistant said. "The shop has chosen you,"
She grabbed you by the wrist and dragged you across the aisles of costumes that seemed to grow stranger the deeper you go inside the store. They were your usual human skin suits at first. The ones that looked like your common pedestrian. Then they changed to animals, mythical creatures, and monsters you could scarcely describe. You look in front of you. The assistant is speeding across the room for someone walking with minute strides. Then you saw it. Her legs stretched to supernatural lengths then reeled back to her skirt. For four of your steps, she took one. But before you became distressed about it, you two stopped abruptly in front of a rather large door. It reached the floor to the ceiling. What little remains of its red paint are now scraping off. A rusted keyhole seemed to suck your key when you pushed it inside.
You pushed the door open and all you saw is the darkness. But beyond that was dim light shining upon a monstrosity you could only see in legends or scary movies. There lies a naked humanoid figure. Even at this distance, you know that thing isn't human. Its face, or the lack thereof, is nothing but a single sheet of bruised skin. It has dark indentations where the eyes and mouth should be. Its naked body is surprisingly clean, except for a sheen of its own sweat. Its head looked in your direction, the skin on its neck sagging along.
"What the fuck is that!?" You ask.
"A skinwalker. A matriarch of its clan. Exiled and now burdened with servitude to this establishment, which in turn, servitude to you." She replied.
Suddenly, another light shone across the room. It's another door with another woman that looked like the one beside you. She has someone in her arms. It's a guy that's about your age. Even though smaller and thinner, the assistant's doppelganger has a tight hold over him. She did not even budge nor grunt in pain when the guy punched her in the face.
"Let go of me!" The man screamed. "I'll pay. For the love of god, I'll fucking pay!"
"I am afraid you've expended all of your time, Andrew." The doppelganger said with no life behind her voice. She pushed Andrew towards the skinwalker and locked the door behind her. He lept back and pounded the door. But all he touched is air. The door was gone as if it wasn't there in the first place. He looked in your direction, and for a moment, there was a look of freedom in his eyes. That all washed away when he sensed something grab him by the ankle. It was the skinwalker and despite the lack of face, you know that thing is hungry. The skinwalker, although frail and thin, had a tight hold over him. Its hands with blood crusting underneath the nail grabbed his face like paper and yanked it off his head. You expected blood and viscera to come out. But there was nothing. Andrew's blood-curdling screams were immediately cut off. The skin of his head lay strewn on the floor, its mouth wide open from its final scream. The rest of his skin followed. What remained is a blank mannequin. And before long, the skinwalker retreated to the far corner of the room.
"It feeds on pain," The assistant stepped inside the room and picked up Andrew's skin. She returned and hung the skin on a hanger, then placed it on a moving rack. "As long as that key belongs, it won't feed on you."
The assistant closed the door, and bid you with a smile. The moments after that were a blur. You found yourself standing back at the mall entrance. But the mall, as you should have known, was already abandoned decades ago. You're standing on an empty concrete jungle with nothing but the key and the knowledge of its use. The key can be used on any door, and all those doors will lead back to the monstrosity you've just seen. Push anyone else inside and they're fed to the monster. Their skin opens for you to use.