The Omega Continuum is an extradimensional plane that traverses the entire multiverse without intersecting it.
The Omega Continuum contains the destructive force required to end the multiverse once it has run its course and, at the same time, to create a new one, it exists in balance with the Q Continuum, which contains the ultimate creative power of the multiverse that counters Omega's destructive power.
The Omega Continuum is the source of Omega, a much more powerful version of the Omega molecule, which is less powerful since its a synthetic molecule corrupted by the boronite used to create it. (VOY novel: The Eternal Tide)
The balance between the Omega and the Q continuum was corrupted thousands of years ago by an extremely advanced species called the Anschlasom who entered it in an effort to explore the universe in search for other sentient species, this led to the creation of thousands of openings to the Omega Continuum throughout the multiverse, an unexpected consequence of this event was an alteration of the Q Continuum at its creation which led to the development of sentient life within it, the Q.
The Omega Continuum was sealed in the 2380s by the crew of Voyager at the cost of Seven of Nine's life in the timeline in which it took them twenty years to get home. These actions were undone by Admiral Kathryn Janeway when she went back in time to shorten her crew's return to the Alpha Quadrant.
In an effort to repair itself, the Omega Continuum generated Afsarah Eden when Miles Jobin and Carson Taller encountered a portion of it in the Beta Quadrant fifty years earlier so that she could encounter it once again in the Delta Quadrant and repair the damage done to it by the Anschlasom and in the process erasing the Q from the history of the multiverse.
Fortunately for the Q the balance existing between the two Continuums meant that for a child of the Omega there was a child of the Q that was supposed to help seal Omega away from the rest of the multiverse and this child was in fact Junior, the son of Q.
Together they gave their life to stop the Omega Continuum from ending the multiverse before its time and in the process spared the Q from being erased from the multiverse and freed Jobin and Taller from their imprisonment within the Continuum. (VOY novel: The Eternal Tide)
Like the Q Continuum, the Omega Continuum can't be processed by humanoid minds and requires those who enter it to perceive it in a familiar way, those who perceive it in its true nature are condemned to slowly turn insane.
An interesting aspect of the Omega Continuum is that it records the history of the multiverse as it evolves throughout it, for this reason Afsarah Eden, a child of the Continuum, was able to access such huge library of knowledge whenever her quantum state aligned with its true nature.