Ragnarok is the furition of the contract made between Charles Ze Britannia, Marianne vi Britannia, C.C. and V.V. where they would destroy the gods and unite all humans, living and dead into one being. This plan was thwarted by Lelouch vi Britannia when he used his Gaes to compell the gods to keep existing and to keep time moving. This resulted in the death of both his parent.
The concept of Ragnarok is very similar to the thrid impact of Neon Gensis Evangelion, both in the idea and the way it is carried out, a plan where a secret cabal of the worlds elit and powerful work to unite all of humanity into a singel intety with the purpose of saving it.