Subject One is/was a scientist who long ago swallowed his own potion, figuratively, and unintentionally became a beast, a powerful and nearly immortal Dreadgod. He’s imprisoned in the Labyrinth beneath Sacred Valley. We first hear about him in book 2, Soulsmith, when Lindon finds some of his research notes in the Transcendent Ruins.
Subject One doesn’t recall his birth name — he’s been more beast than man for too long, a Dreadgod consumed by endless need, raw hunger. He goes by many names. The Devourer itself. The Slumbering Wraith. Subject One. “The Father of Hunger” (Reaper, ch 1) He invented hunger madra. He needs hunger aura.
Subject One is (was) a scientist. The details are still a bit unclear, but it seems that his research team attempted to manipulate dreadbeasts so they could leave behind a Remnant, to be used in Soulsmithing. Instead, they unintentionally created Dreadgods. So., about 2500 years ago, roughly, Dreadgods came into the world, the result of research and experimentation conducted in the Labyrinth.
According to Fisher Gesha, the notes Lindon found in book 2 in the Transcendent Ruins explain a little more:
These notes reference an origin for this madra, a single source from which they got all their samples. They were trying to breed sacred beasts that left Remnants of this aspect, but they never made it. At least, not by the time these notes were written.” (Skysworn ch 7)
Subject One and his fellow soulsmithing scientists also studied the problem of corruptive hunger aura, an unnatural substance left in the wake of a Monarch's greedy, needy craving for more power, more adulation, more ... everything. Hunger aura corrupts everything it touches as it roams the world, mangling Dreadbeasts, for one example.
His research into hunger aura and dreadbeasts led to his invention of hunger madra. To test his theory, he put the powerful and tempting hunger binding into himself, becoming the first test subject, and unintentionally became a beast, hungry, powerful, and nearly immortal. He’s the first Dreadgod, and the progenitor of the other four Dreadgods. (see Reaper ch 21)
Because he’s a monstrously hungry and powerful beast of a man, hard to kill and nearly immortal, he’s kept bound in the Labyrinth in the power-suppression field covering Sacred Valley. However, despite his imprisonment, he still has the power to attack anyone who enters his domain, using his powerful hunger madra.
Subject One is locked away deep in the Labyrinth, sometimes accessed via the Ancestor's Tomb near the Heaven's Glory sect of Sacred Valley. In order to keep Subject One from escaping and to keep its four Dreadgod "children" from reaching him, a script circle with the diameter of a hundred miles was enscribed deep underground that draws the power out of any living being up to about the level of a low gold or jade, because the prison can only be opened with the Authority of a Sage or higher.
Subject One seems capable of creating hunger madra monsters to protect his domain deep in the Labyrinth. It seems capable of creating Hunger constructs that look like skeletal humanoids with huge gaping mouths that attack anything with madra in it. It can also shift around the rooms and corridors in the labyrinth, like a moving maze.
Subject One dies in book 10, Reaper
See New Wiki
For much more info, go to the new wiki at Abidan Archives, Subject One