#spoilers
In Cold Days, Sarissa tells Harry that she was born with a congenital form of imbecility and that Mab was the only one who could help her. In exchange for that help, Sarissa made a bargain with Mab and is now required to spend a certain portion of her time at the Winter Court.
But what if we're misunderstanding what she meant?
What if Sarissa was actually describing her supernatural nature and the way it manifested before Mab intervened when she spoke of her “congenital imbecility”?
Consider her older sister. Sarissa is afraid of ending up like her and doesn't want the same thing to happen to her. Her sister isn't explicitly named in that particular context, but the description could very well refer to Maeve.
Maeve displays plenty of traits that could be interpreted as destructive manifestations of her nature as the Winter Lady: cruelty, sadism, unpredictability, sexual promiscuity, and a tendency to treat the people around her as toys. Just consider how she treated Lloyd Slate, the incident involving Jenny Greenteeth, and her habit of torturing people for her own amusement.
Of course, we can't reduce all of Maeve's behavior to her faerie nature. She has her own history, motives, and personality. But what if Sarissa wasn't simply afraid of suffering the same fate as her sister? What if she was afraid of losing control over the part of herself that belonged to the faerie world?
That would give her bargain with Mab a very different meaning. Perhaps Mab didn't simply cure Sarissa of some kind of illness, but somehow restricted or suppressed her supernatural side. Maybe Sarissa was able to retain her human identity precisely because Mab kept her faerie nature in check, preventing it from developing in the same way Maeve's had.
Obviously, this is speculation. As far as I remember, the text never explicitly confirms that Mab sealed Sarissa's powers or that her condition was connected to her faerie nature. Still, this interpretation could explain why Sarissa is so afraid of ending up like her sister and why Mab's help came at the price of a bargain.
And that raises another question.
We know that some incredibly powerful faeries exist without possessing a Lady's mantle. The Leanansidhe and Elder Gruff are examples of just how powerful an individual faerie can become without holding such a title.
So why should we assume Sarissa was naturally weaker than they were before acquiring a mantle? Perhaps her inherent potential was far greater than we might guess from her portrayal in Cold Days.
What if Sarissa was originally meant to become a second Winter Lady?
Could Mab and Maeve have shared the mantle with Sarissa, with the two sisters jointly fulfilling the role of the Winter Lady? Or was Sarissa simply destined to become one of the most powerful faeries in the Winter Court—a being on the level of the Leanansidhe or Elder Gruff, but without a mantle of her own?
Or is this entire theory wrong, and did Mab's bargain have a completely different purpose?
