The Key to Hell is an item that can lock or unlock the gates to Hell. Whoever possesses this key has dominion over the underworld.
The key was originally possessed by Lucifer Morningstar. When Lucifer decided to retire as ruler of Hell, he expelled all demons and damned souls from Hell before locking Hell's gates and handing over the key to Hell to Dream of the Endless. From Dream, the key went on to Duma, once control of Hell was handed over to two Angels, Duma and Remiel, then Cristopher Rudd until it finally came to Elaine Belloc.