In a famous soliloquy in Act III.1 of Julius Caesar, Marc Antony predicts that there will be a period of "Domestic fury and fierce civil strife" throughout all of Italy. This is what actually comes to pass. The strife finally terminates in the Battle of Philippi, where the forces of Antony and Octavius defeat those of Brutus and Cassius, thereby obtaining absolute power in Rome. In Antony's soliloquy he expresses his belief that it will be the spirit of the murdered Caesar who will be the cause of all the bloodshed.
"And Caesar's spirit, ranging for revenge, / With Ate by his side come hot from hell, / Shall in these confines with a monarch's voice / Cry 'havoc!' and let slip the dogs of war."
Later, Caesar's Ghost actually appears to Brutus in Act IV.2, and in Act V.3, when Cassius has committed suicide and Brutus is preparing to do the same, Brutus says: "O Julius Caesar, thou art mighty yet. / Thy spirit walks abroad, and turns our swords / In our own proper entrails."