The Mask of Gloom is a slayer mask obtained from Treasure Hunter, or from the Shattered Worlds reward shop for 135 million anima. It is associated with the dark beast slayer monster. It gives two free teleports to dark beast locations each day, although these teleports may only be used if the player has partly completed Mourning's End Part II, or has been assigned to kill dark beasts by a slayer master. The mask also provides bonus Slayerexperience while killing dark beasts and doubles their drops on every tenth kill. The extra experience and drops are lost when the mask reaches a certain kill count, at which point it will be transformed into a helm.
Once per day, while wearing the relevant mask, the next assignment the player receives will be dark beasts. They will still need to go to a Slayer Master that normally assigns these creatures (Morvran or Kuradal) – 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, or when The Raptor during November 2015 has assigned Dark Beasts.
Twice per day, this mask will teleport you directly to a location where dark beasts can be found.
The mask confers 12.5% damage boost against dark beasts (but only while on assignment and if the helm variant hasn't been unlocked).
Killing dark beasts while wearing this mask will grant additional 90 Slayer XP if the helm variant hasn't been unlocked. 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 dark beasts while wearing the mask will give double drops for every 10th kill if the helm variant hasn't been unlocked.
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 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 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.