The skull mask in monster hunter world requires the rare unknown skull and three other bones materials. Aside from looking badass, the skull mask comes with the skill handicraft, which improves the sharpness of your weapon. Some high rank weapons can even be raised to sharpness white with handicraft.
HOW TO FIND UNKNOWN SKULL
This rare drop can be found in the Elder’s Recess. Unknown skulls can drop just like any other bone material in bone piles but make for some of the rarest materials in the game. Bring some patience and keep farming the bone piles in the Elder’s Recess and you will get lucky in the end.
Brutal Bones and Dragonbone Relics can also be found in bone piles in this area. While Dragonbones can also take a while to get, you will probably find them while your farm for the unknown skull.
HOW TO FIND WARPED BONES
The Warped Bones can be found in the Rotten Vale and are a common drop in bone piles there.