Lilith was a Carrionite who, like the rest of her race, was banished to the Deep Darkness by the Eternals.
Biography
In her human disguise, Lilith was a beautiful young woman, but in her true form, she was a hideous witch.
Through the words of William Shakespeare after the death of his son, she and her mothers, Mother Doomfinger and Mother Bloodtide, were able to escape into the universe on Earth in 1598. From their arrival on into 1599, she helped influence in the construction of the Globe Theatre, persuading the architect, Peter Streete, to build the theatre so that it would have 14 sides.
In 1599, the three Carrionites opened a portal to the Deep Darkness, allowing all other Carrionites to enter the universe, but they were stopped by Shakespeare after he created a phrase that closed the portal and trapped Bloodtide, Doomfinger, and Lilith in their crystal ball, which the Tenth Doctor took. (TV: The Shakespeare Code)
References
When Rose-the-Cat asked the Tenth Doctor if Rose Tyler had ever said the "L-word" to him, he mistook her phrase to be a reference to Lilith's name; adding "she only broke one of my hearts and that was completely different!" (COMIC: A Rose by Any Other Name)