The abyssal slayer contract is a reward from Shattered Worlds, purchased from the Shattered Worlds Reward Shop for 500,000 shattered anima.
Contracts are purchased inactive. Activating the contract provides a random Slayer-style task for monsters found within Shattered Worlds. Only one active contract can be owned at a time. While carrying an active contract, the specified monsters grant Slayer experience each kill. Upon completion, the active contract vanishes.
A contract can be used in addition to a normal Slayer task.