The Omega War
Rifts are three-dimensional holes in the space-time fabric, connecting different worlds to each other. Most rifts are fairly small and relatively harmless, though still dangerous to ordinary humans without some sort of artificial force field protecting them.
Larger, more violent rifts are called Omega Rifts. These Rifts either kill humans who pass through them, or transform them into Omegas. Such Rifts have their own gravity wells and can suck in nearby objects or people, and are therefore very dangerous and disruptive – but fortunately quite rare. Any human who passes through an Omega Rift alive emerges on the other side with Omega powers.