Luna, the Goddess of Spores, didn't refuse the invitation. Like Medivh, she became a member of the Light Alliance.
After the Light Covenant was dissolved, the Gods of Light were no different from other God Rank powerhouses. They were all free of restrictions.
Medivh and Luna hadn't joined the church before because its affiliation would have bound them, preventing them from starting plane wars and expanding their territories.
Now that restriction was gone, and the invitation came from the Light God Alliance. How could they refuse?
The Goddess of Spores wasn't a faith deity. Unlike Hestia, the Goddess of Fire Prayer, she didn't need to stay in her divine kingdom and refine the power of faith.
The Holy Remains Plane had a Spore Jungle, a node in Luna's Energy Network. While she was in the Holy Remains Plane, she could still control the network and drive the plane's leaps in evolution.
The Origin Plane was difficult to create, but planes below the Origin Plane could evolve into Legendary Planes relatively easily.
Luna, the Goddess of Spores, planned to elevate every plane in the Wrynn Alliance to a Legendary Plane.
Only by expanding the Energy Network could the Sam Plane at its center make another leap forward.
The Thirteenth Rank was only the Lower God level, not the end. Luna, the Goddess of Spores, still had a long road ahead.
Luna lived in the Holy Remains Plane and would occasionally seek out Medivh for some feeding, strengthening her identities as the Myconid Sovereign and the Mother of Ten Thousand Dragons.
Being with Medivh was a win all around: she could enjoy herself, promote the propagation of Myconids, and breed Myco-Dragons on a large scale.
Medivh and Luna didn't lose themselves in pleasure. They kept an eye on the changing situation in