What is the significance of the appearance of the Ghost in the first scene of "Hamlet?

What is the significance of the appearance of the  Ghost in the first scene of "Hamlet?

The ghost in Hamlet serves at least a three-fold dramatic purpose. It introduces an element of mystery and fear by creating tragic atmosphere in the play, motivates the entire action of the play by imposing a task upon Hamlet and finally shows up the characters, and produces a kind of moral effect in them.

 The action of the play begins when Bernardo, an officer, arrives on the platform of the castle rampart at midnight to relieve Francisco from his guard duty. Presently, Horatio and Marcellus join Bernardo and identify themselves to Francisco as loyal subjects. When Francisco leaves, Bernardo tells Marcellus that the Ghost has not appeared yet. Then Marcellus explains to Bernardo that Horatio has joined their watch to verify his theory that the Ghost is a hallucination.

Just as Bernardo begins to describe the Ghost's previous visits, it appears. Bernardo notes that the specter is dressed in armor and seems to look like the late King of Denmark. But when Horatio questions the Ghost, it vanishes. It, of course, becomes visible in this scene again after a few moments, but when Horatio commands it to speak, it fades away just as the cock crows.

The appearance of the Ghost at the dead of night, which is dark and bitterly cold, evokes a mood of foreboding and creates an atmosphere of mystery and fear. It mystifies the awe-stricken spectators, Horatio, Marcellus and Bernardo. Although the Ghost resembles the late King of Denmark, it could be a demon. But if it is really the late King's spirit, why has it come back from the dead? "A mote it is to trouble the mind's eye", says Horatio about the Ghost and likens its appearance to the ominous events in ancient Rome before the murder of Julius Caesar.

The appearance of the wordless Ghost is an important dramatic device. Horatio as a scholar and trusted friend of Marcellus and Bernardo, serves as a reliable witness to the Ghost's appearance and as conveyor of this news to his good friend Hamlet. He also reports that Denmark is preparing to deal with the military threat led by young Fortinbras, nephew of the old and senile King of Norway.