Megara

The Megara were micocellular metalic organisims (bio-machines) who upheld the statutes of Galactic law. They were sometimes referred to as Justice Machines.

History
They appeared as floating, glowing orbs. They killed their own creators for contempt of court. They were sometimes assigned to transport prisoners. When two orbs transported Cessair and her servants, she escaped and they pursued her to Earth. (DW: The Stones of Blood)