The demon slayer crossbow is a weapon with increased potency against demonic races. It is dropped by a demon boss, which will show up as part of a Demon Flash Mob at a number of places every full hour, and it is dropped by the Kal'gerion demons in the Kal'gerion dungeon. Although the crossbow is supposedly level 60 equipment, when used against non-demons it has the accuracy of a level 57 weapon.
Jagex has indicated that you will "get a boost to damage dealt and experience gained while fighting demons for each piece of the gear you equip. A full set of the gear makes the crossbows better than their dragon equivalents when fighting demons".