Answer: Osric
Explanation: In Act V, Scene II, of "The Tragedy of Hamlet, Prince of Denmark", the character that reveals to Hamlet that the poison rapier is the king's treachery is Osric, a foolish courtier. When Hamlet is telling Horatio that he had replaced the sealed letter and in that way is safe from death, Orsic enters. After a while he tells Hamlet that Claudius has poisoned the rapiers and the swords that are going to be used in the fight between Hamlet and Laertes.