The Mask of the Dagannoth is obtained from Treasure Hunter, or from the Shattered Worlds reward shop for 63 million anima. It provides several bonuses for players looking to train their Slayer skill, including bonus XP, extra damage, and double drops.
After killing 1000 Dagannoths, the mask will become a Helm of the Dagannoth. The helm has similar benefits, but does not provide bonus XP, extra damage, or double drops.
Once every 3 days (day resets at midnight UTC), while wearing the relevant mask, the next assignment the player receives will be dagannoths. They will still need to go to a Slayer Master that normally assigns these creatures – 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. This effect requires Horror from the Deep. The cool down period of this ability may be checked via the "Check Kills" option on the item's right click menu.
Killing Dagannoth while wearing this mask will grant 25 additional Slayer experience.
Killing Dagannoth while wearing the mask will grant additional drops for every 10th kill. This does not double effigy but does double charm drops.
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.