In latin this means one’s self; the ego.
In CCC it’s described as “One’s Desire.”
Egos are made from the fragmented personality of the girls who were made into sentinels, and are essentially no more than incomplete “copies.”
They are sentient beings who seeped out of the cracks of the dungeon floor, and like shields, attacking them is useless.
After making it past their Secret Garden they are destroyed, and they return to their original body.
They are the heart and core of each floor, so as you search around it’s not difficult to encounter them. However the real problem starts from then. How do you deal with an Ego, the true essence of one’s desire? Will you be able to overcome them?

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA