Question

What is the meaning of this last dialogue of Antony,'Mischief,thou art afoot, Take thou what course thou wilt!'???

Answer 1

Antony has spurred the crowd to chaos, and now he will watch what they do.

Antony has just given a speech in which he stirred up the crowd against the conspirators.  Remember that before Antony spoke, Brutus spoke.  Antony made sure that the crowd knew that Brutus gave him permission to speak, and then he made sure that the crowd knew exactly what happened to Caesar.  He points out exactly how they betrayed Caesar.