The raptor key is an item used to unlock the Raptor's chest next to Burthorpe bank. Each of the monsters that drop the key pieces require level 96 Slayer to kill, in addition to being on the respective Slayer task in order for them to drop. Each part has a 1/256 chance of being dropped.
Once all four key parts are obtained, simply use one of the pieces on another. This will create the key, with a dialogue message stating "You combine the four key parts into a reward key for the Raptor's chest of slaying."
Following the Patch Notes on 6 February2017, Raptor key pieces were made approximately 150% more common, and the rewards contained within the Raptor chest were reduced to compensate for the increased drop chance.