The Inner Key of Oz, a special key in a way similar to the Key to Oz, is a magical tool which has the power to separate a person into two doppelganger versions of him or herself. The soul of the victim is split into two full-embodied selves, one being the "good" side and the other the "bad" or "dark" side. The good side lived on by all the original person's qualities and virtues, while the dark one by all his or her vices and flaws, being pure evil, selfish and vile. Because of that, neither are capable of indulging in the counterpart morality that the other embodies, no matter how amoral or morally gray the deed may be.
The Key, however, still leaves both parts magically connected by a physical repercussion effect, which makes any bodily injury inflicted upon one to immediately reflect on the other, which means if one gets killed, the other will die as well.
If one of the parts manage to get the Inner Key again, the spell can be undone by that part apparently and simply holding it against one's chest.
In There's No Place Like Home, Sam and Dean find that Charlie Bradbury has returned from Oz and is going after the person who killed her parents in a drunk driving accident, something that was covered up. Charlie proves to be violent and goes as far as torturing people and slashing the Impala's tire to keep the Winchesters from following her. However, it turns out there are two Charlie's: Good Charlie and Dark Charlie. Good Charlie explains that in Oz, she and Dorothy fought a war to free the land from evil and were losing, so Charlie made a deal with The Wizard of Oz in which the Wizard used the Inner Key of Oz to split Charlie into her two halves, which allowed Dark Charlie to single-handedly win the war, though she did a lot of bad things in the process. Afterwards, Dorothy and the Wizard became the new leaders of Oz and Good Charlie followed her dark self to Earth to stop her from doing more bad things and to lock her up forever.
Sam and Dean convince Good Charlie to reunite with her dark half who can't be killed without killing Good Charlie, so Sam and Good Charlie travel to the Bunker to do research on the Key to Oz in hope of repairing it, for Dark Charlie broke it. Their research leads them to a former Man of Letters named Clive Dylan, the man who had discovered the Key to Oz and became stranded there. Clive reveals then he is unable to help them repair the Key, and also that he is the good side of the Wizard, who is his evil, power hungry and dark self, the two having been split by a coven of witches by the time he got stranded in Oz. In order to stop the Wizard and Dark Charlie, Clive shoots himself to force the Wizard to come to Earth to heal them both. At the same time, Dark Charlie visits Russell Wellington, the man who killed her parents and convinces Dean to let her talk to Russell. Dark Charlie claims to forgive Russell, then kills him and flees.
After visiting Dean in a bar, she steals the Impala and follows Dean to Clive's house where the two fight and Dean breaks her arm and brutally beats her. The Wizard of Oz arrives and starts choking Sam to death, while Good Charlie feels the pain of every hit Dean lays on Dark Charlie. In order to stop the Wizard and at Clive's insistence, Good Charlie reluctantly shoots Clive, killing both him and the Wizard. Sam stops Dean from beating Dark Charlie to death and he is able to use the Inner Key of Oz again, taken from the Wizard's body to reunite the two Charlie's. Two days later, Charlie, unable to return to Oz, decides to help Sam and Dean find a way to remove the Mark of Cain and forgives Dean's actions even though he refuses to forgive himself.