Omega-level threats are the greatest threat known because they can affect things on a cosmic scale, be it by eradicating entire intergalactic civilizations or putting the very fabric and stability of reality at risk. This threat level is the broadest in its scope, allowing for a variety of actual ability potencies. Restraining such threats requires a legendary act by beings of lesser threat levels, or an intervention of another Omega being/artifact/power. Generally speaking, though, Omega-level threats are considered unstoppable forces of nature/the cosmos/whatever, and are better tricked, negotiated with, or avoided altogether than directly opposed, as undertaking the latter can easily result in the dangers meant to be avoided.
Containing such beings requires the power of a sun, act of god, or similarly extraordinary expenditure of cosmic power or influence, feats usually only available to other Omega-level beings or incredibly advanced civilizations.