Once per day, while wearing this mask, the next assignment the player receives will be Jellies. They will still need to go to a Slayer Master that normally assigns these creatures (Chaeldar, Vannaka)– 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.
The mask confers the same damage boost against jellies (but only while on assignment) as the Slayer helm.
Killing jellies while wearing this mask will grant additional Slayer XP equivalent to the Slayer level required to fight them (in this case, 52). 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 jellies while wearing the mask will grant double drops for every 10th kill. (This also counts for charms.)
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 999,999 kills.
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.