The Deep Ones are sinister piscene humanoids, alien in origin and ancient beyond reckoning, appearing as a hostile, sentient aggressor species encountered in Etrian Odyssey III: The Drowned City.
Created by the Abyssal God and led by the Eldest One, the Deep Ones feed off human vice and weakness, and are empowered by emotions like rage and fear. Dedicated to corrupting and eventually subsuming humanity entirely, the Deep Ones represent an unimaginable threat to all that live near Armoroad, and represent perhaps one of the greatest threats yet seen.
Their arrival is what set off the Calamity, and as the Abyssal Shrine sank to the ocean floor, it became the lair for these Deep Ones and they began to control the ocean floor, with sighting seen throughout the world. However, the Abyssal God was sealed underneath the Yggdrasil Labyrinth, while the Eldest One transferred a part of his power to Princess Gutrune, leading to his imprisonment by the Abyssal King.