The Deep Darkness was a dimension that existed outside N-Space. During the Dark Times of the universe, the Eternals banished the witch-like Carrionites and the Hervoken to the Deep Darkness.
Three Carrionites escaped from the void and in 1599 opened a gateway between Earth and the Deep Darkness. All trapped Carrionites began to enter Earth but the gateway was closed by the brilliant words of William Shakespeare with the help of the Tenth Doctor and Martha Jones. (TV: The Shakespeare Code)