There are a great many mysteries in the universe, worlds and artifacts whose origins are unknown. Therbre is a world that at some point thousands of years ago had been inhabited by a race that had constructed sophisticated cities of granite with incredible precision. No one knows who they were or even where the name Therbre came from. There is no trace of the Therbreans; no biological trace remains of them or any artifacts of their existence. In fact there are no animals inhabiting the land masses.
There are many theories; biological warfare that utterly destroyed organic mass was one idea but that does not explain any trace of artifacts. Another is a mass migration but that does fit all the answers either.
A few years ago four immensely powerful beings reached the planet and it was discovered the world was some sort of massive engine, each city a component. The nature of the engine was unknown but ships from light years away detected unprecedented vast fluctuations in regards to sensor data. The event terrified officials and now the planet is under strict quarantine. No one is allowed on the world under pain of instant arrest and incarceration.