This story is a sequel to From Darkness to Dawn

Celestia and Luna, along with their step father De Writer, return from the Sunrise Isles, where they have been trained in the use of their Creator Magic for correcting the heavens.
They find that a cult of unicorns led by the "Sunlord" are claiming credit for the fixing of the heavens. The Cult Leader opposes them.
Gaining some followers, they return to the big ledge where they sheltered a hundred years ago. There, they build the earliest parts of Fortress Canterlot.
They succeed in defeating the forces of the Sun Lord, founding a kingdom with their creator Skyglow, Titan of Life Creation as the titular Queen and accepting to themselves the role of Princesses.