The Mask of the Abyss is a reward from Treasure Hunter, or bought from the Shattered Worlds reward shop for 135 million anima. It requires 50 Defence to wear. It provides several bonuses for players looking to train their Slayer skill, including bonus experience, extra damage, and double drops every tenth kill. After killing 900 Abyssal demons, the mask will become a Helm of Warping. The helm has similar benefits, but does not provide bonus experience, extra damage, or double drops.
Upon death, the mask does not appear under your gravestone and is turned into 17,500 coins. However, if lost, it may be reclaimed from Diango for 25,000 coins.
Once per day (day resets at midnight UTC), while wearing the relevant mask, the next assignment the player receives will be abyssal demons. They will still need to go to a Slayer Master that normally assigns these creatures (Sumona, Duradel/Lapalok, Kuradal or Morvran) – 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.
Twice per day, this mask will teleport you directly to a choice of locations where abyssal demons can be found.
The mask confers the same damage boost against abyssal demons (but only while on assignment) as the slayer helm.
Killing abyssal demons while wearing this mask will grant additional Slayer experience equivalent to the Slayer level required to fight them (in this case, 85). This is given regardless of the current assignment the player has, and in addition to assignment and/or contract experience. The experience awarded by the mask can be boosted by items.
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 experience and double drops effects, and turns the mask into a helm. The kill count continues to be tracked beyond this point, up to 1,048,575 on the item's tooltip. However, only up to 999,999 can be displayed to others with the "check kills" option.
A right-click option announces total kills of that creature. The announcement of total kills is not seen in the chat bar but is seen above your character by those around you. 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 experience and double drop effects to the mask.