The Heroes have vanquished Gadreel, but it is a bitter-sweet victory. The sun that now shines in the Virtous Sphere has been stolen from Realmsphere, leaving the Heroes' home without its source of light and warmth.
Satine knows that inside of her hides the power to be the sun. She knows that the only way to harness this power is to feel love, true love, unconditional love. That love Gadreel was never able to harness, that last component he was never able to obtain nor replicate in his attempts to create a new sun . The love she bears Ayrin.
And so the Heroes travel through the Flow back to Realmsphere, finding it cold and dark. The Pyre has been gone for several weeks, dooming the living beings of the sphere to an icy death. Only those able to access powerful magic, or living in the deepest recesses of the Underdark, are able to survive.
There's only one way to save Realmsphere. The crew of the Curse of Strahd and the Conudrum gather on the modron craft to say their goodbyes to Ayrin and Satine. Captain Anomander leaves his ship to the star-crossed lovers, and then they all watch the black ship sail away with them, to reach the center of the sphere, where once was the Pyre.
Hours go by, then suddenly, a spark. A light. And then an explosion of fire and radiant energy, the very birth of a new sun! The Heroes bathe in the sunlight, tears flowing down their cheeks, safe in the knowledge that their beloved friends will always be with them.
Up there, far away, shining their light on all of Realmsphere.