The exciting force is an incident that starts a struggle. In the case of Hamlet (1601), this struggle is first introduced by Claudius.
The play starts when the ghost of Hamlet's father shows up in Denmark and talks with Hamlet. He tells his son that he has been murdered by Hamlet's uncle, Claudius, who has inherited the throne after marrying Queen Gertrude. Before leaving, the ghost orders Prince Hamlet to avenge his death by killing Claudius. In that way, the exciting force that starts the conflict is caused by Claudius, who, driven by an ambition for power, murders King Hamlet and becomes the enemy of the main character of the play.
