Plutarch:
History is an important source for ghosts as reported haunts are found in ancient 
literature. Respected and revered writers, such as Homer, Vergil, and Seneca, left 
accounts of spirits in their stories. Shakespeare based his Roman plays upon ancient 
history, mythology, and legend.  

It is known that plays written about Julius Caesar existed before Shakespeare but 
no connection exists between these and the Julius Caesar play (Brockett 109).   Instead, Shakespeare appears to have gone directly to the source through Thomas North, who translated the works of 
the Roman historian Plutarch into English.  The first edition, dedicated to Queen Elizabeth, appeared in 1579 with a second edition published in 1595.  

The core value of the ghostly encounter between Brutus and his evil genius exists here and is transferred into the play.  This is an exert of the aftermath of Caesar's assassination and describes the extent of Brutus' haunting: 

Brutus being to pass his army from Abydos to the continent on the other side, laid himself down one night, as he used to do, in his tent, and was not asleep, but thinking of his affairs, and what events he might expect. For he is related to have been the least inclined to sleep of all men who have commanded armies, and to have had the greatest natural capacity for continuing awake, and employing himself without need of rest. He thought he heard a noise at the door of his tent, and looking that way, by the light of his lamp, which was almost out, saw a terrible figure, like that of a man, but of unusual stature and severe countenance. He was somewhat frightened at first, but seeing it neither did nor spoke anything to him, only stood silently by his bed-side, he asked who it was. The specter answered him, "Thy evil genius, Brutus, thou shalt see me at Philippi." Brutus answered courageously, "Well, I shall see you," and immediately the appearance vanished. When the time was come, he drew up his army near Philippi against Antony and Caesar, and in the first battle won the day, routed the enemy, and plundered Caesar's camp. The night before the second battle, the same phantom appeared to him again, but spoke not a word. He presently understood his destiny was at hand, and exposed himself to all the danger of the battle. Yet he did not die in the fight, but seeing his men defeated, got up to the top of a rock, and there presenting his sword to his naked breast, and assisted, as they say, by a friend, who helped him to give the thrust, met his death.

To read the entire account, please see: 
http://ancienthistory.about.com/library/bl/bl_text_plutarch_caesar.htm