The Darkness

Before there was light, before there was God and the archangels there wasn't nothing, there was the Darkness. A horribly destructive amoral force that was beaten back by God and his archangels in a terrible war. God locked the Darkness away where it could do no harm, and he created a Mark that would serve as both lock and key, which he entrusted to his most valued lieutenant -- Lucifer. But the Mark began to assert its own will, revealed itself as a curse and began to corrupt. Lucifer became jealous of man, God banished Lucifer to Hell, Lucifer passed the Mark to Cain, who passed the Mark to you -- the proverbial finger in the dyke. – Death, 10.23 Brother's Keeper

The Darkness is a destructive and amoral force that existed before the existence of God. When God came to be, He along with the archangels battled the Darkness in terrible war, the Darkness was eventually defeated, and trapped. To serve as the lock and key to this cage, God created a Mark, and gave it to his most trusted archangel -- Lucifer. The Mark eventually began assert a will of it's own, and began to corrupt Lucifer, which led to his contempt for humanity and fall, leading to the Mark being passed onto Cain and eventually Dean Winchester. To remove the Mark of Cain, without having passed it on to another, would break the lock and unleash the Darkness onto the Earth.