In Act 5, of "Hamlet", Laertes tells Hamlet of Claudius' ___?
I think that this refers to how Laertes tells Hamlet that it was the king (Claudius) that poisoned the drink. After Gertrude falls ill, she claims, "The drink! the drink! I am poison'd." Hamlet isn't sure who put the poison there, and is about to launch a man-hunt for the villian when Laertes steps in and admits, "The King, the King's to blame". So, to fill in your blank, it could be "treachery", "betrayal", "poisoning of the cup". Once Laertes confesses who the true villian is, it isn't long before Hamlet FINALLY dispatches of the king and enacts the revenge he's been pining about the entire play.
And here he is doing it, shortly before he dies of having been cut with the poisoned sword:
In thee there is not half an hour of life.
The treacherous instrument is in thy hand,
Unbated and envenom'd. The foul practice
Hath turn'd itself on me. Lo, here I lie,
Never to rise again. Thy mother's poison'd.
I can no more. The King, the King's to blame.