The Skeleton Key is a magical key that can open any lock.
After receiving the key, MK and Mei used it to go to several places in the city and have fun until MK realizes what the key is really capable of. However, Red Son steals the key from MK and escapes by using his Inferno Truck. Although, MK and Mei goes after Red Son and manages to get the key back until Princess Iron Fan arrives and fights off MK and Mei, allowing Red Son to get the key again and escape with her.
Later on, the Demon Bull King uses the key to open the tomb and sees what's inside it.