In Shakespeare's Julius Caesar, Sardis is not a character, but a place (located in what is present-day Turkey). In the play, however, this is where Brutus and Cassius and their armies set up camp when they are forced to flee Rome.
After the murder of Caesar, Brutus addresses the crowds gathered for Caesar's funeral, in Act Three. Brutus, who felt honor-bound to remove Caesar from his throne before he could harm Rome by becoming king—a dictator—has defended his actions (and those of his co-conspirators) to the satisfaction of the Romans gathered. However, Antony gets up to speak, having promised he would not cast blame at Brutus and the others. However, when Antony is done, the crowds have changed their minds and Brutus and the others must flee. The crowd screams:
Revenge! About! Seek! Burn! Fire! Kill!
Slay! Let not a traitor live! (III.ii.214-215)
When Brutus and the others have rejoined each other, it is in Sardis, at their camp.
To pursue Brutus and the other conspirators, Antony and Octavius would have had to travel some distance. However, only in defeating Brutus and the others could civil war be ended and Rome united once more.