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.
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.