Mithril Hatchet is stronger that Black Hatchet. It can be used for combat or to cut down trees. It requires an Attack level of 20 to be equipped, and a Woodcutting level of 21 to be used. It can be bought from Bob east of ::home for 520 coins.
Mithril Hatchet can be made with the Smithing skill at level 51 using one Mithril bar.