The following is my entry into the Crystal Spheres contest.
The Ceyparis system is an unusual star system. Ceyparis is known as the Iron Sun, and the system is the remnant of a once-thriving star system. Following a system-wide calamity, the surviving Ceypari have adapted to their new, unique situation.
Ceyparis itself is a perfectly smooth spherical body, composed of dense metallic material. It is highly magnetic, and rotates at a tremendous speed. The magnetic field of Ceyparis is strong enough to dominate any magnetic field on the various planets. Circling Ceyparis is a large, dense ring of highly energized gas particles, known as the Burning Crown. The entire ring has super-heated due to friction, and ignited. It now provides the heat that sustains the Ceyparis system.
Beyond the Burning Crown is the Wind Shell. The Wind Shell is a stable shell of breathable air, with frequent strong winds. The Wind Shell balances precariously between the pressure exerted by the Burning Crown and the gravity of Ceyparis. The Wind Shell also encompasses small asteroids. The Wind Shell is home to one fragment of the Ceypari, who have adapted to the airy realm by evolving sail-like wings. There are also a number of mysterious structures arranged at regular intervals throughout the Wind Shell. The inhabitants avoid these structures, and prevent others from disturbing them.
The first normal planet is Asifor. Asifor is a small planet, about 5500 miles in diameter, which lies just outside the Wind Shell. It was a thriving world that suffered tremendously during the calamity. The surface is a rugged badlands, punctuated by frequent geysers of hot water. Below the surface, a network of caverns and tunnels originally held most of the water that now exists on the surface. The world still has a complex natural ecosystem, which encompasses both the surface and the caverns beneath. The Ceypari living on Asifor harvest the ecosystem's resources, exporting food and natural materials.
Hulior is the next planet in the system, a gas giant 45000 miles in diameter. Hulior radiates a low amount of heat, and it has three habitable moons - Wertel, Kasvic and Mipunn. The moons are economic rivals, competing with each other for dominance in the system. Each moon has a small fleet of ships for exploration and trade. Skirmishes are common, though the moons avoid open warfare.
Boja is the furthest planet in the system. It is bitterly cold, but a few inhabitants still brave the harsh temperatures to explore the small world. The ancient Ceypari left remnants of their civilization on Boja, which their descendants are eager to recover.
The surviving inhabitants of the system - both Ceypari and native flora and fauna - have developed unique adaptations to the intense magnetism that pervades the system. Many creatures use magnetic pulses to communicate, while some ferrous plants absorb magnetic energy to grow.
For a science fiction setting, Ceyparis could be a somewhat unusual neutron star or brown dwarf. The most likely calamity would be a supernova, which wrecked the original star and created the air shell. The pre-calamity Ceypari civilization commanded highly advanced technology. They devised a mechanism to ignite a stable fusion reaction in the Burning Crown to provide heat and light to the system once the star itself died. They also created the floating objects in the Wind Shell to keep it stable, and terra-formed the various planets that survived.
For a fantasy setting, the calamity can be more unusual. Different fantasy races might inhabit each of the various worlds, linked together by their common pre-calamity history and culture. The mysterious structures of the Wind Shell might hold mighty magics, or be the resting places of fallen gods.