This story is alluded to in the Odyssey and the Aeneid, but only Apollodorus tells it in full. He wrote, probably, in the first or second century A.D. A dull writer, but less dull than usual in this tale.
These twin brothers were Giants, but they did not look like the monsters of old. They were straight of form and noble of face. Homer says they were
Tallest of all that the life-giving earth with her bread ever nourished, Handsomest too, after peerless Orion alone.
Virgil speaks chiefly of their mad ambition. He says they were
Twins, huge-bodied, who strove with their hands to destroy the high heavens,
Strove to push Jupiter down from his kingdom supernal.
They were the sons of Iphimedia, some say, others, of Canace. At all events, whoever their mother was, their father was certainly Poseidon, although they went generally by the name of the Aloadae, the sons of Aloeus, their mother’s husband.
They were still very young when they set about proving that they were the gods’ superiors. They imprisoned Ares, bound him with chains of brass, and shut him up. The Olympians were reluctant to try to free him by force. They sent the cunning Hermes to his assistance, who contrived stealthily by night to get him out of his prison. Then the two arrogant youths dared still more. They threatened that they would pile Mount Pelion on Mount Ossa and scale the heights of heaven, as the Giants of old had piled Ossa on Pelion. This passed the endurance of the immortals, and Zeus got ready his thunderbolt to strike them. But before he hurled it Poseidon came begging him to spare them and promising to keep them in order. Zeus agreed and Poseidon was as good as his word. The twins stopped warring against heaven and Poseidon felt pleased with himself, but the fact was that the two had turned to other plans which interested them more.
Otus thought it would be an excellent adventure to carry Hera off, and Ephialtes was in love with Artemis, or thought he was. In truth the two brothers cared only for each other. Theirs was a great devotion. They drew lots to decide which should first seize his lady, and fortune favored Ephialtes. They sought Artemis everywhere over the hills and in the woods, but when at last they caught sight of her she was on the seashore, making directly for the sea. She knew their evil purpose and she knew, too, how she would punish them. They sprang after her, but she kept straight on over the sea. All of Poseidon’s sons had the same power: they could run dry-shod on the sea as on the land, so the two followed her with no trouble. She led them to the wooded island of Naxos, and there, when they had all but caught up with her, she disappeared. They saw instead a most lovely milk-white hind springing into the forest. At the sight they forgot the goddess and turned in pursuit of the beautiful creature. They lost her in the thick woods and they separated in order to double the chance of finding her. At the same moment each suddenly saw her standing with ears pricked in an open glade, but neither saw that back in the trees just beyond her was his brother. They threw their javelins and the hind vanished. The weapons sped on across the empty glade into the wood and there found their mark. The towering forms of the young hunters crashed to the ground, each pierced by the spear of the other, each slaying and being slain by the only creature he loved.
Such was the vengeance of Artemis.