Brightness Reef

A million years ago, the advanced Buyur civilization held sway on Jijo. Then the Buyur abandoned the world, leaving it to rest and restore its ecological balance. Now the vast civilization of the Five Galaxies uses patrols, guardian machines, and sanctions of law to prevent resettlement until the planet is once more ready for civilized life.
But over the centuries it has been resettled. Groups of sapient beings, fleeing persecution or neglect, have ignored the laws and evaded the patrols, landing in secret. After years of warfare and mistrust, a new society has evolved on forbidden Jijo, based on tolerance and respect among its six intelligent races: the mysterious hoon, the crablike qheuens, the centaurlike urs, the wheel-borne g'Kek, the multiple-personality-bearing graeki, and last of all to land on Jijo, humans from Earth, bringing with them a new technology allowing civilization to blossom in the wilderness. They all now live in hiding under canopies of camouflage, in dread of Judgment Day, when the Five Galaxies will certainly discover their illegal colony.
Then the unthinkable happens. A strange starship arrives in Jijo's skies, landing near the settlers' holiest place. The passengers' appearance is familiar, their manner friendly. But do they bring long-feared judgment, or something far worse? Are they willing to destroy the six races of Jijo to cover their crimes?