Demiourgos

Demiourgos, also known as the Hollow Phantasm, is one of the Sept-Terrion and the treasure governing over mirage. The Holy Beast sworn to protect the Demiourgos is Zeit, the divine wolf.

Demiourgos later manifests as the secret last boss Demiourgos of Genesis (創星のデミウルゴス) in the artifact-accessed dimension of, during the Special (True Ending) Episode: Beyond the Illusion.

Background
Demiourgos, the Sept-Terrion of mirage, originally situated itself in the territory now known as Crossbell. Mirage governs over perception and cognition and controls causality.

The people with the Crois family at its centre desired that power to have a deity on earth instead of Aidios, the goddess in the sky. Demiourgos guided mankind by understanding them and everything on earth, and controlling causality. At a glance it may be similar to Aureole, the Sept-Terrion of space which tirelessly fulfilled mankind's desires. Demiourgos, however, was bestowed with dignity and therefore did not make this same mistake. It possessed the wisdom and judgement to properly guide the people without further corrupting them. It would have done so, had its 'heart' not reached its limit.

Understanding and being able to guide all human personalities, actions and irrationality of the world would require 'feelings' (or empathy), like humans have. This led to a gradual deterioration of Demiougoros's 'heart' and it fell ill. Realising that at this rate it would go out of control and harm the ones it ought to protect, it decided to cease the causality of its own existence and disappear from the world.

Revival
Since its disappearance, the Crois family feared the loss of their power and dedicated themselves to creating a replacement for the centuries to follow the Great Collapse. During the 8th century, after a long period of experiments, they ultimately decided on alchemy as their method of choice and prepared a plan that would span multiple centuries to complete. They formed the D∴G Cult and entrusted it with the core, a homunculus referred to as KeA capable of containing the reborn Demiourgos. While the D∴G Cult was tasked with awakening the core, the Crois family themselves focused on banking in order to gather the financial resources necessary for the final phases of their plan.

Ultimately, their creation ended up as the Sept-Terrion of Zero (の至宝), a human-made Sept-Terrion born out of a revived Demiourgos with homunculus KeA at its core. In addition to governing mirage, the Sept-Terrion of Zero also gained control over the two other higher elementals, time and space. This allowed it to alter the history and fundamental principles of the world at will, which it used to change the fate of the Special Support Section. The scene displayed at the very beginning of displays their fate as being killed by Joachim Guenter at the Fort of Sun, which was manipulated by KeA into having Renne and Pater-Mater intervene and allowing the Special Support Section to survive.

The exact extent of this manipulation by KeA is unknown.

Beyond the Illusion -
Demiourgos manifests due to the contradictions induced by the Elysium singularity. She is the final extra boss of the True Ending. After she is defeated, she thanks the people for freeing her and feels relieved as she can finally go into slumber.

Trivia

 * In Gnosticism, Demiourgos (or Demiurge) is a heavenly being and subordinate to the Supreme Being. The Gnostic Demiourgos is considered to be the controller of the material world and antagonistic to all that is purely spiritual. Other related terms derived from Gnosticism are Gnosis and Pleroma.
 * Demiourgos appears as a key character in.