It is said in the Norse myths that the goddess Frigg made all things swear that they would not harm Baldur. Not animal, nor plants, nor anything of the earth, not even poisons. The gods then amused themselves by sending all manner of things at Balder. Nothing harmed him.
Loki being the trickster that he was went in disguise and found the one thing that did not swear. He took this and gave it to the blind god Hoder and help him shoot this at Balder. Since this didn't swear not to harm Balder it pierced him and killed him.