What plant was used to kill Balder?

The story behind the answer

Mistletoe is correct: when Balder’s death was foretold, his mother Frigg took oaths from every creature and thing never to harm him—but she overlooked the “harmless” mistletoe. Loki fashioned a dart (or spear) from it and tricked the blind god Höðr to throw it during a game, and it alone pierced and killed Balder.

This ironic choice—a festive kissing plant causing the gods’ most beloved death—makes it memorable: mistletoe = the “kiss of death.” The tragedy set events in motion toward Ragnarök. Even when Hel agreed to free Balder if all things wept, one figure (often Loki in disguise as Thökk) refused, keeping Balder in the underworld. In nature, some mistletoe is poisonous and parasitic—an apt symbol.