The demon slayer circlet is a helmet specially tuned for fighting demons. It is dropped by a demon boss as part of a Demon Flash Mob, or from Kal'gerion demons. There will be a message in the chatbox describing the location of the Demon Flash Mob when a player is level 100 combat or higher. This message can be toggled off at the Doomsayer in Lumbridge.
Wearing the demon slayer circlet while killing demons on a slayer assignment will give a 1% increase to Slayerexperience gained during the task. Equipping a full set of demon slayer armour will give an 8% total increase to Slayer experience gained while on the task. Wearing the circlet also grants a 2% experience boost in the chosen combat style, as well as 4% damage boost against demons.