Graz'zt
Graz'zt, The Dark Prince
Tall, thin, and strong, with skin like obsidian, yellow fangs, pointed ears, and six fingers per hand; this demon lord is as beautiful as he is terrible.
He is believed to have conquered three layers of the Abyss.
He is sometimes associated with the devil Asmodeus.
Ezri had a dream where six fingers featured and ominously preceded an event where a stone giant from outside Gracklestugh was driven to madness by discovering he had come to have been cursed with six fingers. In the Whorlstone tunnels, the source of the curse was believed to be found.
While atop a plateau in the Underdark, Kais heard strange voices declare "The Dark Prince is here!"