The Well of Many Worlds

Hunt Stage 3. Professor Gousse Windcaller has the Well of Many Worlds and a principle for using it: the Well can only be opened onto a plane that shares a real elemental or environmental resonance with wherever it's opened from on the Material Plane. Reaching the Great Library — which holds records Eldoria's own archives can no longer keep, including whatever the Silent Ascendency is racing to erase there first — means finding three such resonant sites and crossing the Plane of Water, the Feywild, and Limbo in turn before the final crossing into the Library itself. Unresolved from the Curse of Strahd arc: Baba Lysaga has not forgiven the party for Strahd's death or the theft of her hut's animating gem, and has bought herself a way to intercept them mid-journey.