A special slayer contract can be obtained from Erskine in Edgeville while tasked with a Slayer assignment for a monster found in the Wilderness. The number of monsters in the contract is the same as the one in the Slayer assignment. It tracks how far along you are in your contract with him. When working on their Slayer assignment in the Wilderness, their kills count towards the special slayer contract, gaining extra 20% Slayer experience each time that they kill their assigned Slayer monster.
A special slayer contract can be lost if the player is killed by another player.
You cannot have more than one Special slayer contract at a time. So, if you have one from Markus, you must complete it and hand it in before you can get one from Erskine.
If you have already killed at least one monster in your slayer task, you will be unable to obtain a special slayer contract.