Ragnarok is an ability specific to Arbiter. It is among his most destructive battle techniques. It is based upon a combination of Thaumaturgy and the power of Order, and activated with his greatsword, Laevateinn.
Arbiter will create an energy ring of variable size around any point of space. Once the ring is complete, a sphere forms around it. Arbiter can draw power from everything living and non-living within the sphere, regardless of his physical location. The sphere can be quickly formed around a planet, but will take much longer to form around a larger object, like a star; a ring around a galaxy would take numerous hours- therefore, it would in most circumstances be impractical. (Stars, powered by nuclear fusion, can grant Arbiter phenomenal boosts in power, and galaxies, powered by black holes and containing countless stars, even moreso.)
After any time, Arbiter can shatter the sphere of energy with his greatsword, releasing catastrophic bursts of power upon everything within the sphere, although when using the power of Order, Arbiter can direct these energy bursts toward a specific target.
Enemies can easily pass thorugh the sphere itself, meaning that (1.) their energies cannot be drained, and (2.) they won't be affected by Arbiter's final move. Arbiter still can draw power from the sphere whether he is inside it or out, though.
Arbiter hates to use this attack- it reminds him of his days as a heartless destroyer of worlds.
Ragnarok is the name of the Norse version of the apocalypse. At the end of Ragnarok, in some variations of the myth, the sword Laevateinn would shatter the dome of the heavens, like how Arbiter uses his greatsword (also called Laevateinn) to shatter the sphere he draws power from.