The story is very loosely based on Greek mythology, combining elements of various myths. In the myths, Theseus was actually the founder of Athens (the capital of Greece) and son of Poseidon (Greek god of sea and Zeus' brother), a hero who killed the Minotaur (a creature with the body of a man and a head of a bull imprisoned in a labyrinth, depicted here as a man with a helmet made of wires and horns) with the help of princess Ariadne whom he later married. Hyperion was the name of one of the 12 Titans. There were only 12 titans in the myths and they were immortal and unkillable. There was a war in heavens and the Titans did lose. Zeus then threw them in Tartarus, which is not a mount(ain) but a part of the Hades (the Greek underworld with minor similarities to Hell) to dwell there for all eternity. Epirus Bow doesn't exist in any mythology, but it might be inspired by Heracle's/Hercule's bow which was used by the Greeks to beat Troy in the mythological Trojan war. The movie's character of Theseus might be partially inspired by Hercules, since Heracle's was a son of Zeus and is the only hero who got ascended to the Olympus to rule with the Gods. Phaedra was the sister of princess Ariadne, and the second wife to Theseus. She was not a seer. Acamas was the son of Phaedra and Theseus, but wasn't a seer. Theseus was murdered in his old age, but not in a battle and he did not ascend. Hoplite was the ancient Greek title for soldiers in ancient Greek states. Heraklion is a capital of the Greek island of Crete and there were never any military formations with that title. Sibyl was the ancient Greek title for prophetesses. There were twelve Olympian gods and goddesses and they were immortal and unkillable as well.
When he was editing the film, Tarsem Singh would set his action to a guide track from the TV mini-series The Pillars of the Earth (2010). He felt that that score seemed to blend in very well with his vision so, when it came to choosing a composer for Immortals, Trevor Morris was the natural choice.
Henry Cavill, who is naturally hairy, had to shave his torso for the film. But he said that shaving had its advantages as he was able to see all the muscle definition that he had achieved for this role.
Director Tarsem Singh told Henry Cavill after their first meeting that if he got the role of Theseus, he would have to develop an "eight pack", not a "six pack". Cavill trained for six months and developed his abdominal muscles into a full "eight pack" which was then displayed throughout the movie in numerous shirtless scenes. He brought down his body fat percentage to 6%, which he subsequently maintained throughout the shoot.
Henry Cavill did not do any weight training for his preparation for this film. His physique was achieved though a a tabata style training regime consisting of only body weight exercises. This training methodology was chosen because the goal was to not put on much mass but to develop a very lean, highly defined physique.
The trivia item below may give away important plot points.
During the final fight sequence between Theseus and Hyperion, Theseus does what is commonly referred to in Mixed Martial Arts as a "Superman Punch". Henry Cavill who plays Theseus, also plays Superman in Man of Steel (2013).