The off-hand demon slayer crossbow is a weapon specially tuned for fighting demons. It is dropped by Kal'gerion demons or a demon boss, which will show up as part of a Demon Flash Mob at a number of places every full hour.
Although the crossbow is supposedly level 60 equipment, when used against non-demons it has the accuracy of a level 57 weapon.
Wielding the demon slayer crossbow while killing demons on a slayer assignment yields a 1% increase to Slayerexperience gained during the task — 8% if wearing demon slayer armour. Using the crossbow also grants a 2% experience boost in the chosen combat style, as well as 4% damage boost against demons .