The demon slayer torso is level 60 Ranged equipment that is a rare drop from the boss of the Demon Flash Mob and as a drop from Kal'gerion demons. According to Brother Celerity, this armour existed in the Third Age. It was blessed by Saradomin and given to demon slayers to increase their capabilities towards demons.
Wearing the demon slayer torso 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 torso also grants a 2% experience boost in the chosen combat style, as well as 4% damage boost against demons.