In the Destiny Crown Designation game, there are actually various preparations that need to be made before going to each Lostbelt, especially the first decisive battle and the last one. In chapter 2.5.6, who is the final master? The following is Let’s take a look at the master introduction at the end of fgo265.
Regarding the identity of Subject E, I had always thought that Subject E was an alien. Now combined with this information, Subject E should not be considered an "alien" in the traditional sense, but should be somewhere else on the earth. Being in a high position, humans conducted inhumane analysis and experiments on their gods, and finally suffered their retribution.
The reason why we understand this is because subject E has the qualifications to become a master, and the servants who obey him are all from the earth. Although the heroic spirits have their own grief and anger towards the history of pan-humanity, they will not rebel for an alien life, then It can only be that this life itself belongs to this planet, but it is different from humans.
The true identity of subject E, according to the Satan in Amakusa's mouth, may be just a nickname. In the current setting, the possibility is relatively high, it may be Earth UO, which is currently the best choice to explain the current situation. The young professor said that he has a clue The qualifications to rebel in the history of pan-humanity, or the fact that he is in a position where he must do so, and has such obligations and responsibilities, all show that he is a being that is higher than human beings, but is inseparable from human beings. If the transition to a clean slate begins here, then probably only UO on Earth has this ability.