A Bonecrusher is a Dungeoneering reward which can be bought from the rewards trader for 34,000 Dungeoneering tokens and is a members only item. It requires level 21 Prayer and 21 Dungeoneering. However, A Dungeoneering level of at least 62 is required for the 34,000 tokens needed to purchase the item.
The Bonecrusher's use is to automatically convert bones dropped from monsters or bones from hunting into Prayer experience. The bonecrusher will have no effect on ashes dropped from demons, also no effect on long/curved bones commonly known to drop by giants. It needs to be in the player's inventory for the effect to work and bones will not appear as a drop since it was already crushed and converted to prayer experience.
The Bonecrusher will crush all bones that creatures drop, not just their 100% death drop.