The Ancient Norse Mythology wasn’t codified until the 13th Century or so, but its stories are timeless

The Nine Realms

In Pre-Christian Norse cosmology, there are nine realms tied together by the great tree Ygdrasil. A few of them are often debated, but the following is a well-accepted rendition from norse-mythology.org

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Midgard, the world of humanity
Asgard, the world of the Aesir tribe of gods and goddesses
Vanaheim, the world of the Vanir tribe of gods and goddesses
Jotunheim, the world of the giants
Niflheim, the primordial world of ice
Muspelheim, the primordial world of fire
Alfheim, the world of the elves
Svartalfheim, the world of the dwarves
Hel, the world of the eponymous goddess Hel and the dead

Ragnarok is the cyclical end times, destruction of the universe, and subsequent rebirth of the gods and the Nine Realms

Ragnarok is essentially preordained. The gods know it’s going to happen and pretty much how but, try as they might to prevent it, there is simply no way to do so. The Death of Baldur is the beginning of Ragnorak as it coincides with the dissolution of kinships among the humans of Midgard.

Listen to Episode 60: TCW S1E16 The Hidden Enemy, The Death of Baldur, and Treachery!