Eros comes upon Antony, who’s philosophizing on nature—exactly what you might expect from a suicidal guy who’s just lost a great battle and is convinced that the woman he sacrificed everything for has betrayed him to his enemy.
Eros weeps, and Antony comforts him with the thought that at least his master can kill himself. This is maybe not so comforting.
Mardian then enters. Antony rages at him, too, telling him he’ll kill Cleopatra for her betrayal. Mardian announces Cleopatra has already taken care of it, that she died with his name on her lips.
Antony doesn’t exactly fall on the ground, but announces that Eros should go to bed, as all their work for the day is now done. He tells Mardian to be grateful that he’s allowed to go safely, as in: "I could have you killed, but I'm not going to."
Even though Antony was just raging against her, we see that the news of Cleopatra’s death is tearing him apart on the inside. He begs his heart to be stronger than his body, or, if not, at least burst open his body as it fills with grief.
Antony says he will catch up with Cleopatra (in the afterlife, presumably) and weep for her forgiveness. Then calls for Eros to return.
Antony tells Eros that Cleopatra has made herself noble by taking her own life. She is, at the end, the sole conqueror of herself.
Thus he tells Eros to kill him. Eros refuses, but Antony reminds him that when he freed Eros (presumably from being a prisoner of war), Eros promised to do anything Antony wished.
Eros readies to kill Antony, but demands that Antony turn away his face before Eros strikes the blow. Antony agrees, and tells Eros to do it now. With his face turned away, Antony misses that Eros has actually plunged his sword into himself, choosing to take his own life rather than his friend’s.
Antony is so moved by the nobility of suicide that Eros and Cleopatra showed that he resolves to kill himself. Antony stabs himself, but finding he has not died immediately, he calls on the guards to finish him off.
The guards refuse which means Antony's going to die slowly and painfully.
Then...Diomedes enters with the news that Cleopatra’s actually not dead.
Cleopatra was just playing a little trick because she was hurt that Antony believed she betrayed him to Caesar, which she definitely didn’t do.
Antony doesn’t flip out, but instead asks that his guards lead him to Cleopatra’s side.