The Mask of Crimson is a slayer mask obtained from Treasure Hunter, or from the Shattered Worlds reward shop for 18 million anima. It is associated with the bloodveld slayer monster. It gives two free teleports a day to the monster, as well as extra Slayerexperience while on a Slayer task of this monster and double drops on every tenth kill. The extra experience and drops are lost when the mask reaches a certain kill count, at which point it can be transformed into a helm.
Upon death, the mask does not appear under your gravestone and is turned into 5,250 coins. However, if lost, it may be reclaimed from Diango for 7,500 coins.
Once per day, while wearing the relevant mask, the next assignment the player receives will be bloodveld. They will still need to go to a Slayer Master that normally assigns these creatures (Kuradal or Duradel/Lapalok) – 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 location where bloodveld can be found.
The mask confers the same damage boost against bloodveld (but only while on assignment) as the Slayer helm.
Killing bloodveld while wearing this mask will grant additional Slayer XP equivalent to the Slayer level required to fight them (50 in this case). 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.
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.