Demiourgos' ability to control perception and knowledge allowed it to manipulate cause and effect. It was able to foresee what happened and would happen in the world, it was capable of guiding humanity and communicating with the other six Sept-Terrion. Since Demiourgos could only guide humanity, it was unable to control the manners in which humanity put that knowledge to use. As a sentient entity herself, Demiourgos realised her knowledge was abused and as a last resort to prevent further misuse, she used her power to end her own existence.
The Demiourgos was originally given to the Crois family. 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, they 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 (零の至宝, zero no shihou), a human-made Sept-Terrion born out of a revived Demiourgos without the 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 Zero no Kiseki 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 not unknown. However, differences between Sen no Kiseki's Prologue scene and the ostensibly same scene in Chapter 5 at Garrelia Fortress, such as the absence and presence of Crow Armbrust and Millium Orion as Class VII members, might be a side-effect of KeA's intervention.