To save a vast dying world, heroes are called from the various worlds of creation and brought to the ring, to save it from crashing into the sun. (World concept inspired by Larry Niven's Ringworld Series.)
Imagine a compromise between a planet and a Dyson Sphere. The "Ringworld" is an artificial ring about one million miles wide and approximately the diameter of Earth's orbit (which makes it about 600 million miles in circumference), encircling a Sol-type star. It rotates, providing an artificial gravity that is roughly equivalent to Earth's gravity through the action of "centrifugal" force. Walls 1,000 miles tall along the edges retain the atmosphere. The Ringworld could be regarded as a thin, rotating slice of a Dyson sphere, with which it shares a number of characteristics.
The Ringworld has a habitable flat inner surface equivalent in area to approximately three million Earth-sized planets. The majority of the surface is land interspersed with shallow, freshwater seas. On opposite sides of the ring are two large deep saltwater oceans, placed in counterbalance to one another. One of the large oceans, known as the "Great Ocean", contains one-to-one maps of all of the inhabited worlds of known space: Toril, Athas, Krynn, Midgaard, Greyhawk, Earth, Mars, Ebberon, Venus, Barsoom, Luna, Midkemia, and all other worlds have analogs within the Ring's "Great Ocean"