He is the leader of the Deep Ones, the alien race who seek to end the existence of the humans. Despite his name and powers, he takes form of a infant Deep One when the player first meet him. Although extremely powerful, before the game took place his powers weakened for some reason, allowing the Abyssal King's army to capture him, and imprison him in the caves below the Deep City. Of his many abilities, one of the most notable ones is being able to spawn Deep Ones from his shadows.
The explorers first meet him on his prison, deep inside the Molten Caves, as a task given by the Abyssal King. He immediately summons a Deep One and a Deep Lady to attack, but those are ultimately defeated. After that, depending on the player's decisions, they may meet him again, and even help him to redeem itself from his past mistakes.
The eldest one is always depicted as a calm being, rarely changing his tone - though he invariably acts as if completely in control; during his first encounter with the party, he regards them with mocking obeisance if they attempt to attack him, regarding them with the same respect a predator might give a mouse: He sees the humans of the party as beneath him. This supreme confidence aside, he still reacts coldly and logically, with almost chilling reasonability.
More emerges if the player attempts to make peace between the Deep City and Armoroad. Perhaps for the first time in his existence, the Eldest One appears to lament his decisions up until this point. His reasoning is intentionally ambiguous: hints are equally placed that the Eldest One had feelings for the Princess and that he viewed these feelings as weakness, and urged the party to purge her corruption as a way of destroying those feelings. Still other hints suggest that he had no feelings for her at all, and simply guided the party in such a way as to bring about his resurgence of power, since his pact with Gutrune left him weak enough for the Deep City's forces to isolate and capture him. Still another hints at genuine remorse. He also wonders if humanity and the Deep Ones can coexist with each other (due to the Princess approaching him without fear), and may end up asking the player's guild for their thoughts.
The interpretation remains up to the player. In the end, it matters little: He fights the party in the form of the Progenitor out of obligation to the Abyssal God, acutely aware of his role in the greater scheme of things, created by the Abyssal God and ultimately sacrificed when necessary. While he shows hesitation in his final battle, and respects the party as champions of their kind, his superior attitude remains. When finally slain, it tells of how the Abyssal God came to Planet Earth. He also dies with satisfaction that Gutrune as able to reunite with her brother Seyfried. As his last words, he tells the player that in order to end the conflict with the Deep Ones, their creator must die.
- He is the first major villain of the series who isn't a human.
- Although the Eldest one is often referred as a male, its often implied that he is, in fact, genderless, or rather, both male and female.