The Ghost is the apparition with the face and armor of the dead king, Hamlet's father. On the walls of Elsinore, in Act I, scene 1, it appears to the sentinels and to Horatio and vanishes without answering when the cock crows. In scene 4, it beckons to Hamlet, who follows it.
In scene 5, it says: "I am thy father's spirit, doom'd for a certain term to walk the night," until its crimes are "burnt and purged away." It reveals that the story of a serpent's sting is false: Claudius poured poison into his ear while he slept in the orchard. It asks for revenge, but orders that the queen be spared. When Hamlet makes his friends swear silence on the sword, the ghost's voice cries "Swear!" from beneath the ground.
The text does not say clearly what it is. Upon first seeing it, Hamlet says: "Be you a spirit of health or a damned goblin." At the end of scene 5, he tells his friends it is "an honest ghost." In Act II, scene 2, he doubts again: the ghost "may be the devil," and he needs proof. After The Mousetrap, he takes "the ghost's word for a thousand pounds."
It reappears in Act III, scene 4, in the queen's chamber, visible only to Hamlet: Gertrude asks with whom he speaks and says she sees nothing. The ghost says it has come to sharpen his almost blunted purpose and reminds him to spare his mother. After this it does not return, not even in the duel.