Urgutha Forka

Urgutha Forka was a vicious demon that once spread a virulent plague in ancient times before the ancestors of the drow fled to the Underdark. The surface elves denied the existence of Urgutha Forka, preferring to blame the drow. The drow, when realizing they were immune to the lethal plague, enlisted Urgutha as an ally.

The saying "been yet not been" was a line from a longer drow tale insulting the surface elves who had lost thousands to a demon they refused to admit existed.