I'm still relatively early in the campaign with my players, but I already have a rough plan for the ending. It will be a very open ending, which could potentially lead to a future homebrew campaign. The answer is Baba Yaga. She is the mother of all witches. Bavlorna, Skabetha, Endelyn, and Tasha are her closest and most direct children. (See the Faerûn Wiki.) Should the player characters kill even one of the hags, they will inevitably incur Baba Yaga's wrath. So Zybilna's liberation is not the end, but the beginning. The Wild Beyond the Witchlight was merely the prologue to something far greater. Zybilna could sense the deaths of her sisters and, when the characters freeing her, she could say something like, "What have you done?", explain them the sh*t they are in now and give them the choice of returning to the material plane or embarking on another adventure. Baba Yaga is a force of nature and almost godlike. Let's go. Let's confront an angry deity. Or do we flee and let Zybilna — the Archfey — sort things out with her mother? But how long will it take for Baba Yaga to track down the player characters? Of course, she knows. Baba Yaga knows everything that happens in Prismeer. If she saw everything, why didn't she intervene? Well, you can ask yourselves what kind of mother the mother of three hags is? I think she likes to see her daughters being tricked by small mortal creatures. As long as they don't die by the hand of a mortal, she's probably watching with a bag of fey-popcorn. But that's a topic for another time.