The Locust Horde is the main antagonist of the Gears of War series. They have lived hidden underground until Emergence Day when they came out of the ground in almost every major human city on Sera. The Horde consists of all sorts of creatures from the mostly humanoid Drones to lumbering beasts like the Berzerker and Brumak down to tiny creatures like the Kryll and Wretches.
At first, not much was known about the Locust Horde aside from that they came out of the ground and seemed bent on the extinction of the Human race. Later it came to light that the Locust are actually fighting a two front war. On one side were the Humans occupying the surface and on the other were the Lambent who were pushing them out of their underground tunnels. It was this war with the Lambent that was the catalyst for Emergence Day. The Locust have been trying to remove Humans so that they could live on the surface, away from the Lambent and escape their own annihilation.
While their technology seems to be on par with that of humans, it appears that most of their mechanical technology is actually adopted human technology. Most of their weapons have been either dropped by humans or appear to be crude machines based on human design (like the Hammerburst). Where they excel is in their manipulation of other creatures. They seem to either be capable of genetic engineering to some degree or an innate knack for taming and selective breeding.
Since the Lambent weren't only rumors until about 14 A.E. (After Emergence) and were certainly not considered a race of their own until then, it is presumed that the Locust will make up the bulk, if not the entirety, of the enemies you will face in Judgment.
