Sumona is a woman living in Pollnivneach, who is actually one of The Devourer's numerous disguises. In the Smoking Kills quest, Sumona asks you to descend into the Pollnivneach well to rescue her sister who has been kidnapped by monsters. After slaying a number of mighty banshees and their mistress, Sumona's sister will destroy an ancient book and disappear.
After the quest, you can speak to Catolax in the well to discover the truth about the quest - The Devourer took on the guise of Sumona to lure you into a deadly trap. Her sister was another disguise, created so she could watch you die fighting the banshee mistress. Because you survived, Sumona is bound by her promise to help you and becomes a Slayer Master, though her assignments are far from safe and her powers mean you forget her true identity when speaking to her.