Forerunner WoMD 2

Thus my father and the Master Builder began to design a series of installations, far fewer in number than the proposed Shield Worlds—what would become the Halos.

By radiating a powerful burst of cross-phased supermassive neutrinos, these installations were capable of destroying all life in an entire star system. Properly tuned and powered, they could do more than that—they could kill all neurologically complex life across whole swaths of the galaxy.
(Halo:Cryptum; Ch.34)

343 Guilty Spark: In reality, it may be significantly more than that. And if the full array was tuned and activated by Installation 00, the harmonics of the overlapping waves would magnify that effect exponentially, cascading to cover every known star system.

343 Guilty Spark: And that only considers the firing of a mere seven Halos! Had the original twelve rings survived to see use...sterilization would spread far further than most Forerunners ever feared we could reach.
(Halo Combat Evolved Anniversay; Terminal 8)

The Builders made the first Ark and the earliest Halo installations ... Large revolving rings, thirty thousand kilometers in diameter, capable of sustaining millions if not billions of organisms on their inner surfaces. Paradise for research in one way - but designed ultimately to destroy all life for hundreds of thousands of light-years around.
(Halo:Silentium; Ch.3)