The Forerunner are a highly advanced humanoid civilization in Bungie's famous video game series: Halo. They were whiped out to extinction many years before the story, and are considered to be Gods to the Covenant religion. Their technology is many centuries more advanced that humans in the games and books, and has been put into use by the Covenant forces, who don't know its full potential. The Forerunner only came into power after the extinction of the Precursors, who were most likely even more advanced than them.
The Forerunner became extinct after having no other choice but to fire the array (Halo rings) to kill off the flood, also killing them. The topic on why they didn't go to their shield Worlds is still under debate.
The Forerunner were very powerful.