|“||The Romans always wanted bread and circuses—food and entertainment! As we destroy their city, I will offer them both.||”|
–Ephialtes, in The Mark of Athena
During the First Giant War, Otis and Ephialtes planned on reaching Olympus by stacking three mountains - Olympus, Ossa, and Pelion - on top of each other. Ares planned on stopping them, but the twins managed to defeat the war god and placed him inside of a bronze jar for thirteen months, until Hermes saved him.
The twins were later defeated by the goddess of the hunt, Artemis. Artemis transformed herself into a deer, which the two wanted to kill. They both took aim with their spears and struck each other as Artemis ran between them.
In the Series
After Otis and Ephialtes were defeated, they began to make a home under the Coliseum in Rome, transforming it into their own hypogeum so they could one day destroy Rome. They also formed a pact with the monster Arachne. For centuries, they aided her in killing or capturing children of Athena who were given a quest to find the Athena Parthenos. The two also performed various stunts in order to gain media attention (normally at Otis' expense), but failed every time.
Prior to the events of the book, the twin giants had kidnapped Nico di Angelo. They held the demigod prisoner in a bronze jar, similar to what they did to Ares in the original myth. They planned to have him die on the Kalends of July, with Gaea returning the next month.
They were forced to fight Percy Jackson and Jason Grace in the Colosseum. Percy and Jason fought the twins, and even though at first, it was completely equal after the two demigods were separated, they were forced to retreat. However, they were soon also able to separate the giants and quickly take down the rather surprised Otis. However, Ephialtes was able to disarm both Percy and Jason and nearly killed them. The Argo II then fired on him and Bacchus struck the Gigantes down after they were almost defeated by the demigods.
It is mentioned by the Titans Hyperion and Krios, who are guarding the Doors of Death, that all the giants who were killed have reformed and returned to the mortal world through the Doors of Death, which includes Ephialtes as well.
Otis and Ephialtes were among the giants assembled in the Athenian Acropolis, as seen in Piper's dream. Otis and Ephialtes fought the Seven Heroes of Olympus in the final battle of the Second Gigantomachy, and were ultimately brought down and killed by the combined efforts of Percy Jackson and Poseidon, with the latter spraying the giant twins out of the Parthenon with a high-powered water blast from his trident. Afterwards, Hades (invisible with his Helm of Darkness) opened up an abyss under the fallen giants' bodies, sending Otis and Ephialtes back to Tartarus.
Ephialtes is shown to be more serious and controlling than his laid back and less serious brother Otis, as he is shown constantly snarling at his brother, as well as ordering him around. While Ephialtes normally comes up with the plans to gain attention, once even making Otis hang inside a huge block of ice over the River Lethe, and once making him run through an obstacle course full of drakons. Normally he is met with failure for one reason or another. He is also shown to be more irrational and sadistic than his brother, since rather than using Nico to lure the seven demigods to Rome, he wanted to kill him in more painful ways: suspending the bronze jar with Nico over a fire, or dissolving it in a pool of acid.
Much like his brother, he plans on taking over Bacchus' spot as the god of parties. However, his parties tend to be very over-the-top and unappealing. He also constantly becomes annoyed at his brother for pointing out his failures for attention or when his brother tries to add something to the plan, such as a ballet number. Normally the only thing stopping him, however, is a lack of time or planning.
Ephialtes is only twelve feet tall, a respectable height for a cyclops, but only half as tall as most giants. He also looks somewhat more human than most of his brothers, since he has two green, yellow-eyed snakes for legs (which he usually conceals under his black pants). His long purple hair is braided with golden and silver coins. He has his ten-foot spear strapped to his back. His looks are almost identical to his brother Otis, the only difference being their hair color.
- Prowess in Battle: In spite of being very short for a giant, Ephialtes is a very strong and skilled warrior, more formidable than his brother Otis. In The Mark of Athena, he successfully fought Percy Jackson, one of the strongest demigods in the world, and even briefly held his own against both Percy and Jason simultaneously and even managed to disarm them. He was, however, defeated shortly after that as the Argo II fired on him knocking him back and defeating him with the help of Bacchus. He is later defeated again in The Blood of Olympus by the combined might of Percy and Poseidon.
- Giant Invincibility: As a giant, Ephialtes can only be killed by a god and demigod together, as his armor and skin are regenerative and will heal during battle at an accelerated rate. However, he can also be killed by other giants, as seen when his brother Otis accidentally did so in Percy Jackson's Greek Gods. He is brought down in The Mark of Athena by the combined efforts of Percy and Bacchus, and in The Blood of Olympus by the combined efforts of Percy and Poseidon.
- Reflexes: Ephialtes has incredible reflexes, enough to deflect a lighting bolt from Jason with his spear in The Mark of Athena.
- Madness Control (presumably): As the bane of Dionysus, the wine god, it can be rightfully assumed that Ephialtes has a certain degree of control over madness although he never used this ability in the series.
- Ephialtes and Otis are the only two giants so far who share the same bane, though that's most likely due to them being twins.
- They are also the only giants who were born to defeat a demigod, as Dionysus was a demigod when he originally defeated them.
- Ephialtes uses the stage name "The Big F". Otis thinks that Ephialtes should rethink that name, but Ephialtes thinks it is perfect.
- As revealed in Percy Jackson's Greek Gods, Otis and Ephialtes are known as the Aloadae Giants.
- Ephialtes probably has low intelligence, due to the fact he doesn't know his name has "ph" and not an "f" and choosing "The Big F" as his stage name.
- In Greek mythology Ephiatles and Otis are demigod sons of Poseidon rather than gigantes.