Midir

Descendant of the Archdragons, Midir was tamed by the gods and set to fight abyssal creatures and defend the Ringed City. The Dark crystalized in his flesh.

Midir remembers, somewhere deep within his soul, a time before the dragon war of the gods of Lordran, and a power that predates the First Flame itself.

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i think its really interesting that in Dark Souls, Dark is the force that makes humans human, and it is both the unstoppable force and the unmoving object. a powerful spell cast by the most powerful person ever to live and master of the opposing force of Light can only barely contain what tiny amount exists in our souls, and not forever. Anyone who comes into contact with it for too long will inevitably be consumed by it, reverting into a mindless and bestial state, physically and mentally. It even corrupts dragons, beings that existed before time as we know it. humanity is a destructive force in the world, but trying to control it and snuff it out doesn't work. After the First Flame sprang into existence and created these forces of Dark and Light and Life and Death, the world was meant to be cyclical. a period of light precedes a period of dark, and meddling with this cycle has thoroughly ruined the world.
maybe the destructive nature of darkness isn't inherent but is instead caused by attempting to stifle it? maybe it's fear of humanity, one's innate nature, that makes it corruptive. idk