The Fatui are the diplomatic and military force from the cryo nation, Snezhnaya, which is ruled over by the Cryo Archon, the Tsaritsa or Barnabas. There are 11 elite heads of the Fatui, known as the Harbingers, who have been granted god-like power by the Tsaritsa. The director of the Harbingers is Pierro,
Then from 1st to 11th: Il Capitano, Il Dottore, Columbina, Arlecchhino, Pulcinella, Scaramouche (former harbinger, now known as Wanderer), Sandrone, La Signoria (former harbinger, dead), Pantalone, There is no official tenth harbinger, and finally Tartaglia.
The Fatui have also created the Delusions, a more dangerous alternative to Visions. Delusions seem to use the remains of dead gods and other powerful entities. The harbingers tend to use visions in combination with delusions, such as Childe (Tartaglia) with a hydro vision and an electro delusion.
These delusions are granted to the user by the Tsartisa herself and only a few outside of the harbingers own one. However the side effects can be very dangerous depending on the user, from absorbing the life force of a human, such as the Inazuman NPC, Teppei, to immediately killing someone like Diluc's father upon use
(or at least leaving him in such agony that Diluc chose to take his life to spare him the suffering).
The Fatui are considered the antagonists of the main story within Genshin Impact alongside the Abyss Order and can often be seen around the game world as battle enemies. All three of the current playable harbingers, Childe, Scaramouche, and Arlecchino also have boss fights. Their main mission within the Archon Quests is collecting all the gnoses of all other Archons. At the time of writing, they have all the gnoses except for the Pyro Gnosis. As for why they are collecting these, it would seem to be, for when "the day of her rebellion against the divine comes at last", according to the Teyvat Chapter Storyline Preview: Travail.
To continue on from the last section, the "divine" mentioned by Dainsleif (the narrator of Travail) is implied to be the Heavenly Principles, who is in a deep sleep upon the floating island in the sky, Celestia.
According to several side or world quests that have a connection to Celestia, many people have made the connection that Celestia is more villainous than originally protrayed. Celestia is seen dropping divine nails onto
civilizations that tend to learn more about the world than they should or are far too advanced. The main example is Khaenri'ah, which is protrayed as an incredibly advanced civilization that worshipped no god, but was destroyed by Celestia and the gods for exactly that.
The current Cryo Archon was involved in Kheanri'ah's destruction 500 years ago, becoming known as the Cataclysm, and as Venti has stated at the end of the Mondstadt Archon quest, the Tsaritsa personality changed and Venti no longer recognized her.
As such, many players have speculated that perhaps the Cryo Archon has deemed Celestia and the Heavenly Principles the villains, and are collected the gnoses to be able to wage war with them.
Furthermore, it is also speculated that many of the archons who has had their gnosis taken are silently in support of the Tsaritsa. While La Signoria was an incredibly strong mage and harbinger, Venti is a god, no matter how weak he may claim, not even a harbinger with god-like power
can best a god at full or even mid power. Similarly, Zhongli, could easily force the Fatui out, however whatever it was that was offered in the final contract, was enough for Zhongli to provide his gnosis, so it may be that he agrees with the Tsaritsa's plan.
Fun fact: The Fatui Harbingers are named after characters in the itallian theatre, commedia dell'arte.
Fatui (plural: fatuus) also translates into "fools" from latin (lol).
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