Pandora's Box is from Greek Mythology. Near the beginning of the world, Zeus gave the first woman (Pandora) to a Titan named Epimetheus, as well as a box which he said must not be opened (this was all done as a sort of punishment to mankind because they were "too happy" after receiving fire from Prometheus). Being female (so the myth goes), she fell prey to curiosity and opened the box, whereupon all the evils it had been containing - illness, war, famine, etc. - rushed out into the world; thus, her inquisitiveness was the sole reason of our unhappy state then and today. She slammed the lid of the box down, but then heard a small voice pleading to be let out. When she re-opened the box, Hope escaped into the world, and so despite all our misery we survive, aided by hope.
Ragnarok was the end of the world as foretold in Norse Mythology. An epic battle was supposed to occur, where the gods in Valhalla (the Aesir) fight against all the monsters and evil things they had held imprisoned up till then - the great wolf Fenris, the fire giants, Loki, etc. Every event that occurs in this battle (the weather, who kills who, and so on) has already been foretold. In the end, almost all will die and fire will scorch the earth, but a new world and a golden age will arise.
To know more, Wikipedia is very useful:
http://en.wikipedia.org/wiki/Pandora_(mythology)
http://en.wikipedia.org/wiki/Ragnarok