The Death of Baldr
Baldr, the brightest of the gods, had dreams of his death. His mother Frigg extracted oaths from all things not to harm him — except the mistletoe. Loki, disguised as an old woman, learned of this and guided Baldr's blind brother Höðr to throw a mistletoe dart at Baldr, killing him [S2]S2 · Snorri Sturluson, Prose Eddac. 1220–1235textOpen source ↗. Baldr's death is the beginning of the end, setting the chain of events that leads to Ragnarök [S1]S1 · Poetic Edda (Codex Regius)c. 1270 (preserves earlier oral poetry)textOpen source ↗[S2]S2 · Snorri Sturluson, Prose Eddac. 1220–1235textOpen source ↗.
The beliefs are pre-Christian (attested archaeologically from the Viking Age); the written sources are Christian-era (1220–1300) and post-hoc. The gap between belief and text is part of what this section presents.
The 'mistletoe' detail and the full sequence are from the Prose Edda; the Poetic Edda has fragments but not the full story [S1]S1 · Poetic Edda (Codex Regius)c. 1270 (preserves earlier oral poetry)textOpen source ↗[S2]S2 · Snorri Sturluson, Prose Eddac. 1220–1235textOpen source ↗.