Mark Antony commits suicide by stabbing himself with is own sword. He believes that his lover and ally, Cleopatra, has already done so and sees no other way to escape capture by his former friend Octavian.
Mark Antony had been a supporter of Julius Caesar and, after Caesar's murder, ruled Rome alongside Octavian and Lepidus in the Second Triumvirate.
Mark Antony's alliance with Cleopatra became a threat to Octavian because of Cleopatra's assertion to have a child from Julius Caesar. Octavian's claim to power depends largely on his posthumous adoption by Casear; a legitimate child and heir of Caesar might have undermined all that.
The death of Mark Antony, as well as the murder of Cleopatra's son Caesarion, ensures that Octavian becomes the sole, undisputed ruler of Rome.