What is the purpose of the scene with the evil spirit in Act IV, Scene III, in Julius Caesar by William Shakespeare?

In Julius Caesar, Act IV, Scene iii, William Shakespeare exposes the superstitious nature of Brutus.  The setting is Brutus’s tent in a camp near Sardis.  The conspirators meet to plan their battle strategy against Antony and Octavius. 

This day has been trying for Brutus.  He has argued with Cassius intensely.  Accusations were thrown back and forth.  Although nothing was settled, the two men know that they need each other.  They make up and drink together. 

The battle negotiations end as does every disagreement between Cassius and Brutus.  Brutus disagrees with the much better and more experienced soldier Cassius; and Brutus gets his way despite the obviously, ridiculous argument that he gives. 

Brutus finally admits what has been troubling him the most.  He has received a message that his beloved wife Portia has committed suicide by swallowing hot coals. 

The hour is late. Everyone leaves to sleep, yet Brutus wants company.  Lucius calls in two men to sleep with Brutus in his tent.  Lucius plays music and sings briefly, but he is also tired and falls asleep. 

Brutus begins reading a book. Suddenly, the candle begins to flicker. Brutus thinks his eyes are playing tricks on him.  Then he sees an apparition. First, he is not sure what he is seeing and asks if it is anything.  Brutus is scared and aloud he says that his blood runs cold.   The apparition stares at Brutus.  Who or what are you?

Brutus: Why comest thou?

Ghost: To tell thee thou shalt see me at Philippi.

Brutus: Well, then I shall see thee again?

Ghost: Ay, at Philippi.

The ghost disappears. Brutus asks if the apparition is some god, some, angel, or some devil,” but finally settles on “they evil spirit.”  What an odd message to give to Brutus? He calls in the other men to see if they have seen or heard anything untoward.  They all swear that they have seen nothing.

Shakespeare never states that the apparition is Caesar.  He allows Brutus to call it “they evil spirit.”  Interestingly, Brutus is now the one who sees the superstitious prophecy.  This is usually reserved for women.  Brutus normally lives by his reason alone.  Emotional outbursts or scenes are not part of his makeup. 

The scene foreshadows the battle at Philippi.  Brutus realizes that the evil spirit is foretelling his death. The events that were initiated by Caesar’s death will continue with more deaths. This supernatural event provides the recognition that Brutus has changed as well.  He has become more like Caesar giving orders and arguing with his generals. Brutus is becoming tyrannical.  

 At first, he killed Caesar, deluding himself that it was best for Rome.  Now, Brutus is thinking only of himself.  It is unclear who the evil spirit represents.  It could mean that it is truly an “evil spirit” appearing to Brutus’s eyes only.  It may be that the Ghost represents Brutus’s own spirit, which has become more evil.  Regardless, it does not sound good for the future of Brutus.