[An entirely inappropriate thrill of excitement runs through Ciel at that intense look, not because he likes flirting needlessly with death (though he does that sometimes too), but because those unnaturally red eyes signify one aspect of their relationship that Ciel has complete control over. His sneer widens, lending a manic shade to his expression.]
...only when my complete satisfaction with your service has been achieved, my revenge has been completed, and I give you verbal, explicit permission to do so. [Specific enough for you, Demon? You will need to fulfill all three of those criterion in order to have your meal.] You will not be allowed to break this contract unless I tell you, of clear and sound mind, that you are allowed to. I will bind you to me- tighty, irrevocably, and I will not allow you to cut my game short. That is your fate, for the course of my life.
[Maybe Sebastian can still break the contract and renege on this order through sheer will, but that just means it'll fall to Ciel to not give him a reason to. And until he does break it, he will obey Ciel's Orders.]