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 Megapolis and have fun until MK realized what the key is really capable of. However, Red Son stole the key from MK and escaped by using his Inferno Truck. Although, MK and Mei went after Red Son and managed to get the key back until Princess Iron Fan arrived and fought off MK and Mei, allowing Red Son to get the key again and escape with her.
Later on, the Demon Bull King used the key to open the tomb and sees what's inside it.
In Monkie Kid