A special slayer contract is obtained from Markus outside of the Slayer Tower. It is obtained by having a Slayer assignment involving a monster in the Slayer Tower when speaking to him. It tracks your progress in your contract with him. It must be carried in the inventory for kills to be counted.
Any creatures killed for the contract give you 20% more Slayer experience than the assignment would normally give. Reading the contract gives an overview of the task assigned and the progress made towards completing it. Checking it gives a short summary in the chatbox.
When the contract is completed, an orange message is given in the chatbox, stating that a reward may now be claimed from Markus. Players can choose between a combat training book, which provides experience in either Attack, Strength, Defence, Ranged, Magic, or Constitution, or a cash reward. The rewards for a Special Slayer Contract depend on the monster killed, and the amount. The table below shows the coins and experience awarded for each monster. For example, a special contract for a Slayer task of 164 Nechryael rewards either 90,036 coins or 17,384 combat experience.