Responses
As the way things stands, the only thing that can null the contract is death. If Sebastian fail to protect fake Ciel, the contract will be broken and fake Ciel will be free. Perhaps Undertaker is trying to kill fake Ciel and then bring him to life afterwards to save him. That is, if Undertaker's intention is to save fake Ciel from Sebastian.
I feel this is wishful thinking and honestly I want to believe this theory, but man, idk
...A part of me is thinking, what if Undertaker and real Ciel are doing this drama to save our Ciel from Sebastian. Maybe... Undertaker knows about the contract and trying to protect Ciel's soul from being eaten, by 'any' means necessary. (He might be planning something, I mean Undertaker can't do harm to fellow Phantomhive... Right?)
I seriously wanna know how the story will unfold.