Monolith

The Monoliths are enigmatic artifacts found on worlds harboring sapient life across the galaxy. Their creators and purpose are unknown. According to the Briari Ecumene, they are the tools of an ancient cycle of tragedy across the galaxy, that has since been defeated. The Briari warn species who discover the monoliths to destroy them at the first opportunity.
Monoliths are normally found on world that currently, or at one time, support sapient life. Discovering one often has a strange cascade effect where it becomes easier to discover more once the first has been discovered.
The Monoliths themselves are a mystery of engineering, they are easy to reproduce but impossible to understand. Made of conventional materials, a particular alignment of these seems to result in a device that manipulates and emits electromagnetic and Holzmaan waves. Their production of 'free energy' as a result of the electromagnetic waves makes them a cheap and easily reproducible source of energy, making it attractive for a species to create more.