This quest is not available in game.
Speak with Cazul in the Cleft of Shadow.
The time has come for you to learn more of the summoning magics. You will learn to summon a succubus, a creature of incredible mental powers. It can lull those of weak minds into its servitude--but have no fear! I feel certain that you will be equal to the challenge.
A succubus can only be summoned when baited with an item that represents great devotion and love. I leave it to you to find such an object.
A suggestion? Speak with Cazul... There seems to be little the ancient warlock does not know.