Evils of Humanity

The Evils of Humanity, also known as the Human Evil and the Beasts of Calamity are the main antagonists in the entire Fate/Grand Order and even the entire Nasuverse as a whole. They were first mentioned in Fate/Prototype by mentioning the Beast of Revelation, the biblical Anti-Christ who served as the overarching antagonist of Fate/Prototype.

Overview
The Human Evil are the ultimate targets of Grand Order, and transcendental enemies that Heroic Spirit Summoning was originally meant to combat, classified as Beast-Class Servants (otherwise known as the Beasts of Calamity), are seven great threats to the continuation of human order. While they have varied motivations and aren't a "group" as such, their defining characteristic is that their existence and motives pose an existential threat to humanity as a whole.

They are existences fueled by aspects of Humanity themselves (thus their name - "Evils of Humanity") and accumulate more power as mankind advances. As such they are transcendentally powerful, and while they're technically "Servants" due to their nature, they are powerful enough to not only survive but manifest themselves without any need of a Master.

Beast I - Demon God King
Main Article: Goetia

Beast II - Primordial Mother
Main Article: Tiamat (Fate/Grand Order)

Beast III-R - Demonic Bodhisattva
Main Article: Kiara Sessyoin

Beast III-R, formerly known as Kiara Sesshouin, is the infantile stage of the Third Beast, representing Desire. It acts as the main antagonist of the SE.RA.PH chapter.

In the world of Grand Order, the human Kiara Sesshouin was a simple therapist with some interest in computer science and an ordinary job working for the Chaldea Foundation on its Seraphix deep-sea platform.

However, in another life, she was a narcissistic and masochistic cult leader devoted to a twisted religion of desire, and a skilled computer hacker who hacked the Moon Cell Automaton with the goal of usurping its omnipotent power and imposing her twisted ideal of love upon the world. She would seize the power of creation itself and ascend to something close to a true demon, the embodiment of "All The Desires Of The World", only to be defeated.

When the Demon Pillar Zepar attempted to find refuge on Seraphix following the defeat of Goetia, it used its knowledge of these "possibilities" to show an ordinary human what she could have been. As a result, one of the Evils of Humanity, once vanquished at the heart of the moon, instead discovers it may be able to walk the Earth at long last, with the aid of one of Chaldea and Marisbury Animusphere's darkest secrets.

Beast IV - Primate Murder
Fou is an animal that appears to be able to freely walk around the Chaldea Security Organization's complex and has a notable attachment to Mash. The protagonist meets it alongside Mash, and it then tends to come along on the various Singularity missions.

He is in actuality, a Cath Palug who belongs to Merlin and in other universes where the Human Order was weaker, would've belonged to Altrouge Brunestud, the ninth of the 27 Dead Apostle Ancestors as Primate Murder, the first of the Dead Apostle Ancestors and ultimate killer of humanity. It is the Fourth Beast and represents the evil of Comparison (as in envying those stronger than you and regretting your own weakness).

Beast VI - Beast of Revelation
The Beast of Revelation is the overarching antagonist of Fate/Prototype and possibly the entire Fate/Grand Order as well. Throughout the story, Fate/Prototype's Arthur Pendragon has been hopping through dimensions to hunt. It has been killed twice, in 1991 and 1999, but it keeps coming back. It's heavily associated with the Anti-Christ in popular fiction and one of its skills is Nega-Messiah. As a result, there have been indications in side materials that if it manifests in full, Nero Claudius would have some kind of relation to it.