What does the ghost reveal that Claudius murdered father?

The ghost of Hamlet's father, King Hamlet, reveals to Hamlet that he was murdered by Claudius, Hamlet's uncle. The ghost tells Hamlet that Claudius poured poison into his ear while he was sleeping in the orchard, causing his death. Hamlet is shocked and angered by this revelation and vows to avenge his father's death. The ghost also encourages Hamlet to be cautious in his pursuit of revenge and not to be rash or impulsive.