1. Merriam-Webster Dictionary
Etymology: Old Norse Ragnarok, literally, fate of the gods, from ragna, genitive plural of regin, gods + rok, fate or course (later rendered as Ragnarøkkr, literally, twilight of the gods).
: the final destruction of the world in the conflict between the Aesir and the powers of Hel led by Loki -- called also Twilight of the Gods.

2. Columbia Encyclopedia, Sixth Edition, Copyright © 2003
      Ragnarok (räg´nerok), in Norse mythology, the doom of the gods. According to prophecy the end of the world, heralded by three severe ice ages, in which human civilization would be destroyed. Then the gods of Asgard, led by Odin, would clash with the devastating forces of evil and chaos, led by Loki and the giants. After a fierce battle the universe itself would be destroyed by fire and a new golden age would appear, ruled by the surviving gods, including Balder.