The god Baldr was the most beloved god of all. His mother, Frigg, traveled around the 9 realms asking every creature, weapon, person, and anything you could dream of to not cause him harm. They all agreed.
The single thing she thought could never hurt him was mistletoe. So she felt she didn’t need to ask it to never harm her son.
All the gods had a celebration knowing Baldr would never be harmed. They decided to have fun with it and threw rocks and sticks and tried to hit him with swords and other weapons. Nothing harmed him.
Until Loki found out about the one thing that could hurt him. He fashioned an arrow from the mistletoe and tricked Baldr’s brother, Hodr, into shooting him with the arrow to be a part of the fun. The arrow instantly killed Baldr and all of the people, plants, animals… everything felt the instant he died. All because of the Err of Mistletoe.