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.
Set bonus (1): Provides 2% additional experience in the chosen combat style, and 4% additional damage against demons per piece worn.
Wearing full armour results in 10% additional experience and 20% additional damage, while wearing full armour and both crossbows results in 14% additional experience and 28% additional damage.