Very little is known about Subject One. It is a ancient being that is being regarded as the father/progenitor of the Dreadgod and Dreadbeasts. It is sealed away in the Ancestoral Tomb of the Heaven's Glory sect of the Sacred Valley.
In order to keep Subject One from escaping and to keep its "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.
We know virtually nothing about its capabilities except that it seems capable of using pure Hunger madra and that it can persumably corrupt other beasts with it creating Dreadbeasts and eventually Dreadgods. It seems capable of creating Hunger constructs that look like skeletal humanoids with huge gaping mouths that attack anything with madra in it.