Otis and Ephialtes are the names of the twin giants that stormed to Olympus in hopes to destroy it along with the Olympians. They were the twin sons of Iphimedia and Poseidon, the Greek god of the seas. Together, they were known as the Aloadae.
Both giants were the twin sons of Iphimedia and Poseidon, the greek god of the seas. However, they were fostered by their stepmother, Eriboea. Like every giant, they have snake bodies/lizard legs for legs, and they have shown to be the shortest in stature next to Orion, being 12 feet tall.
In mythology, they were known to have attacked Olympus in attempt to overthrow the Olympians and in attempts to gain Artemis, for Otis, and Hera, for Ephialtes. Different versions were reimagined by authors. One version was narrated that Apollo was the one who killed the Aloadae before they could grow full beards. Another version of the story, which was stated to be more briefly narrated, was said that they were able to kidnap Ares, the Greek god of war, when Ares charged against them. They stored him in a jar, which weakened over time. After a year passed, Eriboea, the stepmother of the twin giants, exposed where they encased Ares to Hermes. Hermes was able to tell the gods where Ares was, and was therefore rescued.
One account of the story stated that Ares was only to be freed once Artemis was to be handed over to Otis. This made Ephialtes felt envious as Otis was the only one benefitting. Both giants fought. Artemis transformed herself into a doe and skipped along in the woods. The Aloadae, seeing what was happening, readied their spears so as not to let Artemis run away. Instead, when the threw their spears, Artemis vanished and the spears skewered both giants, thus killing them.
In Percy Jackson and the Heroes of Olympus
The twin giants appeared in Rick Riordan's Heroes of Olympus: The Mark of Athena as the main antagonists in the book. They managed to kidnap the demigod son of Hades, Nico di Angelo. However, they were both killed by their Olympian opposition, Bacchus, the Roman god of wine and madness in the Roman Coliseum. The two giants returned in the final installation of the Heroes of Olympus series, The Blood of Olympus, where they accompanied all the giants in a battle against the seven demigod heroes. They were then both killed by Percy Jackson and his father, Poseidon.
The two giants had a minor setback in an another Rick Riordan novel, Percy Jackson's Greek Gods where they were both mentioned and storied in Ares' side of the novel.