The tension begins almost from the start of the scene, as the castle guard changes at midnight. Francisco, the exiting guard, says, initiating a mood of foreboding,
Tis bitter cold,
And I am sick at heart.
then arrives, and we learn from Marcellus, a guard, that he comes to see the alleged "dreaded sight" of an "apparition." The unease ratchets up when the ghost appears, looking like the late king.
Adding to the tension, Horatio sees the ghost as a bad omen, stating,
This bodes some strange eruption to our state.
If that were not enough, Marcellus asks Horatio what's going on with all the shipbuilding seven days a week. Horatio says it is preparation for war, as Fortinbras
is marching toward Denmark to avenge the death of his father at the hands of King Hamlet
Horatio has a bad feeling about the coming war coinciding with the appearance of the ghost; he mentions that the murder of Julius Caesar
and the civil war that followed his death were preceded by supernatural happenings:
The graves stood tenantless and the sheeted dead
Did squeak and gibber in the Roman streets
As stars with trains of fire and dews of blood,
Disasters in the sun . . .
Finally, the ghost appears again but leaves when the cock crows to signal dawn.
There may not be many surprises in the passage except for the second appearance of the ghost, but the play opens with a strong sense of foreboding: a ghost, an army coming closer, a dark night. By the end of the scene, the audience should feel a rising tension about what will happen.