- "From where does that howl come? That is no demon. It sounds more like a poor hungry child."
- - Saint Urbain
The Old One is an entity and the main antagonist of Demons Souls.
The Old One takes on the form of a colossal serpentine creature made out of trees and other plant matter. It is many times larger than the player, the visible part of it being comparable in size to the bridge in front of the Boletarian Palace, where the Archstone of the Small King is located. It appears on a beach in the middle of a vast desert, which is only accessible from the Nexus once the player slays all five Archdemons and talks to the Maiden in Black. It has no facial features other than a circular mouth, inside of which is a passage leading to a small pond which contains the remains of King Allant XII and what appears to be the Old One's "soul" or "brain".
The Old One is, as its name implies, one of the oldest demons in existence, and is nigh-unfathomably powerful. Its mere presence allows the usage of magic in the form of the Soul Arts, but also is the source of the Fog that is in the process of engulfing the world during the events of Demon's Souls as well as the demons actively destroying civilization. The description of the Talisman of Beasts suggests that the Old One is, in fact, the true form of the God worshipped by Saint Urbain and others, which would imply that the Old One is also the source of miracles. The Old One, unlike the lesser demons, cannot be slain in any sense; it can only be lulled into a kind of sleep, and then it can be awoken again by the widespread use of the Soul Arts (according to the Monumental). If the player chooses to leave the Old One's innards while the Maiden in Black lulls the Old One to sleep, the credits will roll and the "good" ending will be awarded, along with the corresponding Trophy. If the player kills the Maiden in Black before she finishes the ritual and submits to the Old One as a servant, the credits will roll and the "bad" ending will be awarded.