When Baldur began to have dreams of his death, Frigg went around to everything in the world and secured from each of them an oath to not harm her son. Confident in Baldur’s invincibility, the gods amused themselves by throwing weapons and any random thing they could find at Baldur and watching them bounce off of him, leaving him utterly unscathed.
Loki, the guileful trickster of the gods, sensed an opportunity for mischief. He inquired of Frigg whether she had overlooked anything whatsoever in her quest to obtain oaths. She casually answered that
she had thought the mistletoe to be too small and harmless a thing to bother asking for such a promise. Loki straightaway made a spear from the mistletoe and convinced the blind god Hodr to throw it at Baldur. The projectile pierced the god, and he fell down dead.
The whole world did indeed weep for the generous son of Odin – all, that is, save one creature.
The giantess Þökk (“Thanks”), generally assumed to be Loki in disguise, callously refused to perform the act that would secure Baldur’s return.[4]
And so the bright god lay in the grave until Ragnarok, the destruction of the cosmos at the end of the great mythical cycle, after which Baldur returned at last to the land of the living, gladdening the hearts of the creatures who filled the new world.[5]
Source:
https://norse-mythology.org/gods-and-creatures/the-aesir-gods-and-goddesses/baldur/