The Cataclysm is a major event of chaos that impacted all 7 nations in Teyvat 500 years ago. This event is also regarded as the Fall of Khaenri'ah, the Dark Calamity, or the Calamity of Darkness.
The primary events that happened in the period is the emergence of the Abyss and the destruction of Khaenri'ah. Abyssal monsters, many that were created by the Alchemist Gold, came out from Khaenri'ah and attacked the seven nations, each nation getting hit severely, such as Natlan and Inazuma. Later, Khaenri'ah would be destroyed by the Archons.
As a result of this event, the people of Khaenri'ah had been cursed. The curse varied from person to person, especially if they were pure-blooded or came from Teyvat, such as Dainsleif who was cursed with immortality, some Kahenri'ahns had turned into abyssal creatures or they would turn into Hilichurls (enemies that can be battled in Teyvat). The Royal Guards of
the kingdom would gain a new form but some managed to retain partial bits of their consciousness. The remenants of the civilization that had turned into abyssal creatures would band together and form the Abyss Order. The exact circumstances surrounding it is still unknown and so there are many theories as to what those circumstances are.
Two of the seven Archons were killed, the Hydro Archon, Egeria, and the Electro Archon, Makoto, with a third archon, Dendro archon, Rukkhadevata, would die from trying to remove the forbidden knoweldge pollution from Irminsul.
Mondstadt would send a expedition into Khaenri'ah to quell the abyss monsters, leading to the loss of several knights such as Rostam (lover of the Crimson Witch of Flames, Rosalyn / La Signoria). It would see the rise of the Crimson Witch of Flames as well as the Bloodstained Knight.
Mondstadt city would then face Durin and Divalin would defeat the shadow dragon, although it led to Divalin being poisoned.
Liyue had monsters come out from within the chasm and Dunyu Ruins. Morax would summon the millelith to defend and evacuate minors, and the bloodshed would lure in the former yaksha, Bosacius, who perished in the battle with many others.
What happened in Dunyu Ruins is unknown and its people would choose to leave the city and escape to Liyue Harbor.
Inazuma would be ravaged by the abyss, and with the help of Istaroth, Makoto is able to lay the seeds of the Sacred Sakura Tree which would absorb the pollution. It would also see the loss of several notable figures, such as Mikoshi Chiyo, the Kitsune Saiguu, Reiznbou, and Takamine the Mistsplitter. Makoto would then be
slain in the battle in Khaenri'ah.
Sumeru would suffer the impacts later with the arrival of Forbidden Knowledge, leading to the death of Greater Lord Rukkhadevata. Places would be ravaged by ruin automatons and riftwolves. Abyss monsters would also appear in the desert.
In Fontaine, an entity known as Elynas would appear and cause severe pollution to the land and seas. He would die by bloodloss and from his corpse came the Melusines. With the death of Egeria, the new archon, Focalors would ascend, and the oceanids that were loyal to Egeria, would leave the nation.
Natlan suffered greatly and is still currently facing constant attacks from the abyss. They were especially vulnerable due to the fact that their nation had the most unstable and shallow-rooted ley lines. The pyro archon of the time would lead the tribes to victory and repelled the abyss deep underground.
6 heroes from the six major tribes would go straight into the hotspot of Abyss activity.
It is unknown what exactly happened to Snezhnaya during the cataclysm but it would see the personality change of the Tsaritsa and the formation of the Fatui.
Many of the theories surrounding the Cataclysm is mostly about why it happened and what did Khaenri'ah do to anger the Heavenly Principles. It has been implied by Dainselif that Khaenri'ah had likely attempted to rebel against the Heavenly Principles. They created automatons such as the Field Tillers (now known as Ruin Guards) that had immense fire-power and combat abilities. Khaenri'ah also likely had some understanding of knowledge that Celestia did not want people to know about. Another theory poses that the Art of Khemia and Gold's work as an alchemist partially led to the fall of the civilization. This is because the Archons led their battle against the abyss in Khaenri'ah once the abyss monsters began to attack their nations.
Fun fact: According to the traveler in the Archon Quest, "A Herald Without Adherents", the traveler states that they remember Khaenri'ah. According to their memories
they awoke soon after the cataclysm occured and with their twin, they attempted to leave Teyvat, leading to the beginning cutscene of the game.
check out more theories!