The Mistletoe Arrow is an arrow crafted from a mistletoe plant by Loki in order to kill Balder. When the god ended up in Helheim, the arrow would reappear in his chest on a daily basis no matter what he did with it, so he just carried it around with him.
Magnus Chase and the Gods of Asgard[]
The Sword of Summer[]
Loki mentions (in a vision of Magnus) how he shaped an arrow from mistletoe and gave it to a blind god named Hod to throw it at Balder which kills him.
9 from the Nine Worlds[]
Nice Doggy[]
Thomas Jefferson Jr. borrows the arrow from Balder when thinking of ways to immobilize Garm. The guard dog eats the arrow and vomits it up when he reaches his cave. Balder retrieves it and goes to wash it off before it reappears on his chest.