The Helm of the Black Demon is obtained after killing 1000 black demons while wearing the Mask of the Black Demon. The helm has similar benefits to the mask, but does not provide bonus XP, extra damage, or double drops.
Once every three days (day resets at midnight UTC), while wearing the relevant mask, the next assignment the player receives will be black demons. 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 the area in Taverley Dungeon where Black demons 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. 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 black demon for few seconds.