The Inner Key of Oz is a magical key made up of steel from Oz. Not much is known about this key, except that it can only be mended in Oz if broken, and that it can release in bodily form the alter-ego of a person's soul.
This key has the ability to split one's soul into two separate beings: one evil, the "dark side", and the other the good one. It appears that while splitting the soul, it also creates a body manifestation for each one, which will reattach by using the Inner Key again. Although separated, their physical bodies are still connected by a magical repercussion effect, and so if one gets hurt, the other will do as well.
The Inner Key can also reverse the soul splitting spell by simply being hold against one of the halves' chest. 
According to the Man of Letters Clive Dylan, the Inner Key originally belonged to an evil witches' coven that kidnapped him soon after he got stranded in Oz. In intention to probably enslave Clive and gain his dark self as an ally, the witches resolved to use the Inner Key on him. However, the plan went wrong, and at some point Clive's evil self managed to overpower and slaughter all the witches. Now aware of what he was capable of, Clive's dark side avowed himself from then on as The Wizard of Oz, and gone on a power hungry quest for Emerald City throne while leaving his good half behind, who was later rescued by fellow Man of Letters L. Frank Baum, Dorothy's father.
A hundred years later, in 2013, when Dorothy was back to Oz along with Charlie Bradbury, a war started; unaware of the Wizard's true intentions and origins, Dorothy and Charlie decided to make a deal with him: by using his Inner Key, (which was stolen by him right after he murdered the witches), Charlie's dark side was released and sent to war, winning it just by herself.
After witnessing, however, the side effects of having one's evil self totally free and independent from the good one's will, Charlie decided to reverse the spell, which arose her dark side's anger, who decided to steal the Key to Oz and go back to Earth, breaking the Key soon after to prevent Good Charlie from asking Dorothy's help.
After reencountering Sam and Dean Winchester, Good Charlie and the brothers managed to discover the whereabouts of Clive Dylan's good self, who was still alive due to his connection to the Wizard. Being made aware of the extension of the problem, Clive revealed his story and decided to help. By taking a gun and shooting himself, Clive obliged the Wizard of Oz to come back to Earth to heal them both. After his arrival, Sam noticed the Inner Key on his belt, and tried to shot him down. However, the Wizard disarmed Sam with a gesture, and started to magically choke Sam to death. Clive then, in a desperate decision, tells Charlie to shoot him again, who reluctantly agrees and murders Clive with a headshot, killing both Clive and the Wizard. Sam then took the Inner Key from the Wizard's dead body, and Charlie finally managed to reattach her alter-ego with herself.
The later whereabouts of the Inner Key were not revealed, but it was very likely safely storaged inside the Men of Letters bunker.
- ↑ There's No Place Like Home