Ragnarok is the fruition of the contract made between Charles zi 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 Geass to compel the gods to keep existing and to keep time moving. This resulted in the death of both of his parents.
The concept of Ragnarok is very similar to the third 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 elite and powerful work to unite all of humanity into a single entity with the purpose of saving it.