In the Palace in Asgard, Queen Frigg found out that her son Baldur is having dreams about his own death
Frigg, worried for her son's safety, got promises from everything on the earth to harm Baldur.
Frigg tells the Norse gods of her work and they excitedly start throwing swords at him which harmlessly bounce off him. Lloki then asks Frigg if she overlooked any thing. She replies mistletoe because she thought it to be harmless.
Loki, armed with his new knowledge, and a spear fashioned out mistletoe tricked a blind god into throwing it at Baldur who promptly died.
For his actions the Norse Gods tied Loki down with his own giant son's guts in a cave with a snake above him to drip poison into his eyes