The foretold end of the Norse gods: fire, flood, a dead sun, then a green world again.
Documented
Written in the Eddas
The Völuspá of the Poetic Edda and Snorri's Prose Edda lay out the sequence: Fimbulwinter, the wolf swallowing the sun, the gods falling in battle, the world sinking and rising renewed. The source texts are secure.
Assessment: Documented as the mythology of record, copied down by Christians two centuries after conversion.
Stories
Twilight by mistake
The popular name is the Twilight of the Gods, from ragnarøkkr. The manuscripts mostly say ragnarök, the fate of the powers. Wagner's opera fixed the moodier word in the world's ear.
Assessment: A genuine philological fork. The doom of the gods reads differently as their sunset, and the sunset won marketing.
Interesting claims
The Fimbulwinter was a forecast of the past
A serious hypothesis ties the myth to 536 AD, when a volcanic dust veil dimmed the sun across the northern hemisphere and Scandinavian tree rings show catastrophic cold. Myth as climate archive.
Assessment: The dust veil is documented in ice cores and tree rings. That Scandinavia kept the memory for five centuries is plausible, argued by named researchers, and unprovable.
Sources and further reading
Poetic Edda, Voluspa
Snorri Sturluson, Prose Edda, Gylfaginning
Bo Gräslund and colleagues on the 536 dust veil and Fimbulwinter
Wagner, Götterdämmerung, for the twilight wording
Worth asking
If the end of the world was edited, what does that make the beginning?