Symbolism, Imagery, Allegory
On the eve of the Battle of Bosworth Field, Richard dreams that he's visited by the ghosts of some of his murder victims – Prince Edward, King Henry VI, Clarence, Rivers, Gray, Vaughan, the young princes, Hastings, Lady Anne, and Buckingham. (Dang. The stage sure gets crowded during this scene.)
Each of the ghosts recalls what Richard has done to them and condemns him to death on the battlefield. Each one chants "Despair and die" (5.5). Yikes!
Oh, and did we mention how the ghosts also pay Richmond a visit (in his dream, of course) in order to give the guy a pre-battle pep-talk? It's a terrifying moment, and Richard's pretty shaken up when he awakens. Check out his reaction:
Methought the souls of all that I had murder'd
Came to my tent; and every one did threat
To-morrow's vengeance on the head of Richard. (5.5.5)
Shakespeare's point? It seems pretty clear that Richard's evildoings have come back to haunt him – literally. (Hmm, was Dickens thinking of this play when he created the Ghost of Christmas Past?)
P.S. Shakespeare really has a thing for ghosts. If you think Richard's supernatural pals are scary, check out the "spirit" that goes around haunting Hamlet.