In the opening of hamlet whom do sentries and Horatio see?

In the opening of Hamlet, the sentries Bernardo and Marcellus, along with Horatio, see the ghost of King Hamlet, the father of the protagonist, Prince Hamlet. The sentinels have been keeping watch and report seeing the ghostly apparition twice before. Horatio, a friend of Hamlet and a scholar, joins them the third night at their observation post to witness and try to understand and communicate with the spirit, thus beginning the events that unfold throughout the play that revolve around the themes of mystery, death, and betrayal.