Gaius Julius Caesar, or just "Caesar", was a recurring character and one of the main antagonists of Xena: Warrior Princess as well as the main antagonist of the episode "Render Unto Caesar" in Hercules: The Legendary Journeys. He was portrayed by Karl Urban.
First Encounter With Xena He was captured by Xena's forces back when Xena was a war lord. She met Caesar and she saw potential in him. At first she wanted to ransom him off to Rome, but then she saw he could be an ally. She seduced the Roman and they made love. Xena's companion, teacher, and former prisoner, M'Lila (who taught Xena the infamous nerve pinch she puts on her enemies so they can give her information while she interrogated them), had a bad feeling about him. Her feelings would be later confirmed when Xena and Caesar would meet again. Caesar betrayed Xena, took her ship, his Roman troops killed her crew, then they crucified Xena and her people after he told her that "She would share a place of honor in his heart...among his conquered." This betrayal scarred Xena mentally so much that it caused Xena to become an even more vicious and blood thirsty war lord until the influences of Borias, Hercules, and her soul-mate Gabrielle, Xena was redeemed.
His war with Xena
Caesar would later on go out and conquer much of the known world and formed the Roman Empire, one that spanned most of Europe, some of the Middle East and most of Northern Africa. However, his battles with Xena made his rule less solidified. He would also come to regret his betrayal to Xena.
One Encounter with Hercules
Caesar would have one encounter with Hercules. Though the two never met face to face, Hercules would with the help of the locals and a Demi-Goddess Morrigan, thwart Caesar in his efforts to conquer Britannia. Caesar ordered his scribe to not have any records of this failed conquest. He wanted his people to be secured in the knowledge that there's no enemy they could not beat. When the scribe wouldn't violate his integrity and refused, Caesar ordered his execution right then and there.
His attempted Ascension to becoming Emperor and Death
He had some violent dreams of Xena killing him in various ways, one was while they had sex, Xena would kill him in his while in a sexual encounter. One day, he woke up to find the demonic spirit of Callisto, who was also an enemy of Xena in his bed chambers offering him a deal. He would become emperor and he would be able to eliminate Xena if he serves her master. She told him to put a bounty out for her capture and death. A lot of people came with false bounties, to which he had executed for their lies. In the meantime, he was making preparations for the Ides of March announcement.
Xena would be captured, thanks to Callisto's help and with Xena's soul-mate, Gabrielle, Caesar had them executed by a crucifixion. Meanwhile, he was about to make his announcement that he would become Emperor, but was brutally stabbed by Roman Senators and his best friend, Brutus. The Ides of March was the day Caesar, Xena, and Gabrielle would all die.
Caesar changing the world and his Ultimate Fate
A few decades after his death, he was in Tarturus (the realm where the dead would go if they live an evil life, much like Hell, only not as bad) he would find a way to escape (thanks to the recent death of Hades during the Twilight of the Gods, security in the underworld became "laxed.") and would find and capture The Fates. He would then alter the loom so that he would have never betrayed Xena.
In this new reality he created, Xena would have become his wife and they would rule Rome as Emperor and Empress. Gabrielle would become a successful playwright, and another one of Xena's enemies in the primary universe Alti would become High Priestess of The Roman Empire. Gabrielle would go to Rome and would entertain the Empire with her play "Fallen Angel." Caeasr suspected a bond between the two. He had Gabrielle locked up under false charges of being an assassin out to kill him. Xena would demand that she would be free after she found out about Caesar's plans and what he done to the world. Caeasr would free Gabrielle only to have Alti track her down and capture them both. Caesar much like he did in the primary universe had betrayed Xena and had her crucified while Caesar made Alti the new Empress of Rome. Caesar would kill Brutus because he distrusted him after he killed him in another life; tying up loose ends.
While Xena was being Crucified Gabrielle would find the Fates and Alti and Caesar would consummate their new union. But like Caesar, Alti was treacherous and lusted for power. She killed Caesar, stabbing him while they were having sex. Caesar's fate is finally revealed; ultimately, he would be stabbed to death. Alti would briefly become the sole ruler of The Roman Empire. However, Gabrielle destroyed the Loom and the world was reverted back to the way it was, leaving Caesar to be dead, for all time.