In Shakespeare's Julius Caesar, Philippi is not a person but a location. This is the place where Brutus and Cassius die: Cassius commits suicide because he believes his dear friend, Titinius, has been taken captive while carrying out a request of Cassius. Brutus commits suicide as well, choosing to die on his sword rather than be taken by the armies of Antony and Octavius.
Plutarch (the Greek historian) reports the actual events of the assassination of Julius Caesar and the ensuing conflicts between Brutus and Cassius against Antony and Octavius; Shakespeare condensed what actually took place.
The battle of Philippi is a dramatic example of Shakespeare's compression of events. In the play, the battle directly follows the meeting of Cassius and Brutus in [IV, ii-iii], whereas it actually occurred more than [six] months later. In fact, there were two battles at Philippi...
Philippi, located in Greece, was the actual location of the battle between the Roman triumvirs and the assassins of Caesar. Shakespeare obviously changed the timing and some of the logistics for the sake of staging the play. Act V begins on the "plains of Philippi." But according to Plutarch, historically...
In the first engagement, Brutus defeated the forces of Octavian and took his camp.
It was in this battle that Cassius actually took his life, believing that Brutus had already been defeated. Though Brutus captured Octavius' camp, Octavius was hiding out in a swamp. Three weeks later, the second battle took place.
After three weeks, Brutus decided to fight so as to maintain his soldiers’ morale and his strategic advantage...Antony defeated Brutus, who consequently committed suicide rather than fall prisoner to the triumvirs.
Defeat of Brutus and Cassius and their armies united Rome once more as the civil war came to an end.
Boyce, Charles. Shakespeare A to Z: The Essential Reference to His Plays, His Poems, His Life and Times, and More. New York: Bantam Doubleday Dell Publishing Group, 1990.