Mistletoe is a tier 4 weapon in terms of statistics, but concerning the stats, it's unbalanced. While the amount of damage of this weapon with 5 point minimal and 11 maximum is remarkable low for slings, the armourpiercing score and the critical hit multiplier both have the maximum score possible for this kind of weapon.
This weapon has the special property to half the accuracy penalty during the nigth, but with night vision, the penalty during the night is negated.
The weapon can be found inside Myrddin Emrys' cave in the Ravine area. While taking some items is considered stealing before the fight, this weapon can be retreived from the small chest in an illuminated part of the cave for free right after meeting with the old druid.
It was said that Loki tricked his blind half-brother Hrodr into killing Baldr with an arrow made of Mistletoe, Baldr's only weakness.