This is purely hypothetical and an opinion I have about events that happened long before the game, none of this should be taken seriously unless you agree with some parts of what you read. Also, I am willing to discuss any of this with anyone if they have questions or have their own opinion
Long before the events of the game, and long before the creation of Ilias and Alipheese the first, humanity had reached a milestone in their advancement in technology, the ability to create living beings. These beings created by humans are Ilias and Alipheese the first, humanity did their best to teach Ilias and Alipheese all they could about morals and emotion, and the difference between right and wrong. Their gained supernatrual powers as a result of this new technology that created them, and rivarly soon grew between the two, always trying to out do eachother, their rivarly soon turned physical and fights broke out between them. As their fighting continued, humanity suffered greatly from the destruction they caused, so they attempted to end it by stopping the two, but it failed and they had no choice but to kill their creations. Their powers had grown too much and they quickly overpowered humanitys attempts to kill them. As their fighting continued the world eventually perished and all of humanity died off.
Realizing what they had done, they both fell into a depression for thousands of years before Ilias had the idea of recreating the world. Ilias recreated the Humans as best as she could, but Alipheese had her own ideas, she created monsters, a mixture between humans and mythological creatures they had learned about from their time on Earth. Ilias, believing them to be horrid creations, wanted them destroyed so the humans could live. The Great Monster Wars was a result, and we all know how that ended up.
Ilias wanted the humans to worship her, having done so much for them, she felt she deserved it. She forbid angels from helping the humans, wanting them to discover and advance on their own like before. She didn't want them to make the same mistakes they did before, but she ended up repeating a mistake the old world had previously made.
Now, we all noticed that Promestein had technology that resembles that of modern day technology, that could be explained through her years and years of research, but it could also be explained as relics and artifacts of the old world that were left behind. Not realizing how to use it, Promestein reverse engineered it and used it to help further advance her research, thus being able to reach the ability to create androids and the Chimeras
Thank you for reading my theory on what happened, I will be updating this post with anything else I can think of, or if anyone has any questions of their own, I will answer it in this post. If you have any questions or theories of your own, feel free to let me know, I am looking for feedback :D