Octavian may have won the battle, and the war, but he hadn’t completely ground Antony into the dust, so he didn’t consider his work quite finished yet. In 30 BC he launched the attack of Alexandria and pretty much pwnd everything. Knowing that all was lost, Antony and Cleopatra decided to make Romeo and Juliet look like a pair of tragic amateurs.
Upon hearing that Cleopatra was dead, Antony drew his sword and stabbed himself. Unfortunately, the message was one day too early. Failing to die promptly, Antony was carried in to Cleopatra who had taken refuge in her own mausoleum. Talk about being prepared. According to Shakespeare, Antony just has time to spiel out some longwinded remarks, and then bites it.
Cleopatra, meanwhile, was not relishing the idea of surrendering to Octavian. After negotiating for her children’s lives, she figured it was better to shuffle off this mortal coil than be taken to Rome and paraded around like an elephant. Ancient sources agree that she got an asp or two to bite her. One modern historian believes that due to the slow-acting nature of snake venom she would have drunk a mixture of poisons instead, but he’s probably just saying that to be a pain in the asp.
With both Antony and Cleopatra out of the way, Octavian was now free to annex Egypt and add it to the Roman Empire. Because he didn’t want anyone else to get their grubby little hands on it, Octavian handled it as his own private territory, and senators were neither allowed to govern or visit it. Egypt was huge politically, producing both grain and treasure. Octavian didn’t want any Egyptian fiasco to cause a famine or otherwise come back to bite him in the asp.*
So in 29 BC Octavian returned to Rome. He celebrated a three-fold triumph over the defeat of Cleopatra (represented in effigy on account of being dead), the battle of Actium, and the battle of Illyium (a flyspeck on the pages of history—he didn’t really deserve a triumph for this one). The doors of the Temple of Janus were closed, for the first time in just about forever, signaling an end to warfare (like that’s gonna last).
*It’s amazing how much use you can get out of this pun.