The Darkness (aka, Amara) is an omnipotent primordial entity who predates both God and Death, and surpasses either of them in power. Only the Shadow is older than the Darkness. When her sibling God came into being, he created the Archangels. Together they sealed the Darkness, being unable to kill her as doing so would destroy reality.
Only a few beings know of her existence or that she is locked away. The others in Heaven and Hell believed her to be a myth. She was recently released by a spell cast by Rowena to free Dean of the Mark of Cain. The Darkness manifested on Earth in a human female body, consuming human souls to regain her power. With Dean's assistance, she reconciled with God and departed Earth.