I, uh, are you okay? It was build up....
Chrollo had the chain around his heart removed during Greed Island. During the Palace Invasion, Hisoka was chasing Chrollo. That's confirmed when Illuma says Hisoka missed the ants because he did some other shit.
Anyways, this bookmark/book he's using is pretty confusing. I never fully understood it, but it's still pretty strong. I wonder if the condition he has set is to announce what it is he can do.