The Mask of Stone is a Slayer mask obtainable from Treasure Hunter. It provides several bonuses for players looking to train their Slayer skill, listed below. When the double drops, damage, and xp boosts have expired (after killing 300 Cockatrices), the mask will become the Helm of Petrification which has similar benefits, but does not provide double drops, extra damage or an XP boost.
It has a requirement of level 20 Defence to be worn. It also counts as a mirror shield equipped, granting the use of two-handed or dual wield weapons.
Once per day, while wearing the relevant mask, the next assignment the player receives will be cockatrices. They will still need to go to a Slayer Master that normally assigns these creatures (Mazchna or Vannaka) – if they do not, the player will be told which Slayer Master assigns them. This does not work if they are in the process of setting up a co-op Slayer assignment.
The mask confers the same damage boost against cockatrices (but only while on assignment) as the Slayer helm.
Killing cockatrices while wearing this mask will grant additional Slayer XP equivalent to the Slayer level required to fight them (in this case, 25). This is given regardless of the current assignment the player has and in addition to assignment and/or contract xp. The XP awarded by the mask can be boosted by items.
Killing cockatrices while wearing the mask will grant additional drops for every 10th kill.
Every creature killed is tallied, while the mask is worn. Each mask has a set amount of kills assigned to it that, once reached, "uses up" the Slayer XP and double drops effects, and turns the mask into a helm. The kill count continues to be tracked beyond this point, up to 999,999 kills.
A right-click option outputs total kills of that creature. If the helm has yet to be unlocked, there’s also a message to say how many more kills are needed to do so.
After the helm version has been unlocked, the player is free to switch between it and the mask version, as a cosmetic preference only – this does not restore the Slayer XP and double drop effects to the mask.