The Helm of Keening is the result of killing 200 Banshees with the Mask of Mourning, which is won from the Treasure Hunter. It requires 15 Defence to wear.
- Once per day, while wearing the relevant mask, the next assignment the player receives will be Banshees. 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.
- Twice per day, this mask will teleport you directly to a choice of locations where Banshees can be found
- A right-click "check kills" option announces total kills of that creature. The announcement of total kills is not seen in the chat interface but is seen above your character by those around you.
- 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.
- 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.
- After the helm version has been unlocked, the player also unlocks an emote in which they can transform into a banshee for few seconds.
- This mask will also act as ear muffs and a face mask (to protect from the Smoke dungeon).
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- patch 23 November 2015 (Update):
- This slayer mask is now hybrid class.