The mithril harpoon is a tool used for harpooning fish in the fishing skill and is capable of catching 50% more fish than a regular harpoon. It requires an Attack level of 30 wield, but a Fishing level of 21 to use. The harpoon can be smithed using the smithing skill at level 53 smithing, using 1 mithril bar. The harpoon may also be gained as a drop from Celestial water warriors as a common drop, but there's a very high leveled requirement to fight these creatures.