An omnion is a particle formed from the entirety of a universe's matter and energy, at every point along its timeline, folded up into a single particle; based on the local universe's or multiverse's laws of physics, it may be a zero-dimensional point particle, a one-dimensional string, a simple particle, or perhaps something entirely different. Omnions might have infinite mass-energy if a universe is infinitely large or has an infinite timeline (or a timeline that can be subdivided with no Planck-time type limit), or they could simply be extremely but measurably energetic.
One way omnions can be created is from The Core of the Omniverse, where universes or other things with very high mass are crushed into the core and shot out, where protoverses and other -verses can be created. Another way is for protoverses to fail, or simply protoverses failing and turning into omnions.
Civilizations capable of time travel might use omnions as convenient methods of storing huge amounts of energy, by collapsing entire universes and storing everything within as an omnion. The total energy of a universe's history can only be exceeded by that of a larger -verse (with a larger "type" of infinite energy), or a replicating power source like metamesonium.