Great Calamity

The refers to two distant events in Hyrule's history related to the resurgence of the Calamity Ganon in.

First Great Calamity
The first Great Calamity refers to the first battle between Calamity Ganon, 10,000 years before the events of Breath of the Wild. Anticipated by the then-highly advanced Hylian civilization, the conflict opposed Calamity Ganon to an unnamed knight designated by "the sword that seals the darkness" and a Princess blessed with an unspecified "sacred power" (illustrated by a Triforce symbol on the Calamity Ganon Tapestry displayed at Impa's home in Kakariko Village), helped by the mechanical creations of the Sheikah tribe: the Guardians and the four Divine Beasts. The Hylian side was victorious and Calamity Ganon was succesfully sealed. It was then prophesied that Calamity Ganon shall break free of the seal, prompting King Rhoam Bosphoramus Hyrule to conduct archeological surveys and unearth the forgotten Sheikah machines, 9,900 years after the first Great Calamity.

Second Great Calamity
The second Great Calamity was a period of mass destruction in Hyrule caused by the return of Calamity Ganon after being imprisoned for 9,900 years. This event led to many deaths and almost every structure across Hyrule crumbling into ruins. Hyrule Castle itself fell under Ganon and King Rhoam was killed during the disaster. With Hyrule Castle having fallen and the army of Hyrule with no leader, they fell back to the fortress that protected Akkala, which also fell and was later known as the Akkala Citadel Ruins. The Great Calamity caused a great split within the Sheikah tribe, as a separate tribe known as the Yiga Clan grew and fought in allegiance to Calamity Ganon. Due to the Great Calamity, the population of monsters in Hyrule significantly increases. It also caused Stalkoblins, Stalmoblins, and Stalizalfos to appear at night.

Calamity Ganon achieved much of the destruction by possessing the same Guardians and Divine Beasts that were used to defeat it 10,000 years before. Calamity Ganon successfully had the lives of the Champions Daruk, Urbosa, Revali, and Mipha cut short through the Blight Ganons. Out of the Champions, only Link survived the Great Calamity, but was placed to slumber for 100 years within the Shrine of Resurrection after being injured while defending Princess Zelda. To prevent the destruction of the entirety of Hyrule and all its people, Zelda spends the 100 years after the Great Calamity inside Hyrule Castle with Calamity Ganon, keeping the demon in place within the castle. This sets the tone for the events of The Legend of Zelda: Breath of the Wild, which take place 100 years after this event.

The arrival of the Guardian Terrako from the future in which the events of occur produces an alternate timeline where the events of the second Great Calamity unfold and end quite differently. In this timeline, King Rhoam survived the attack on Hyrule Castle and the Four Champions are able to be saved. Being led by Princess Zelda--who succeeded in activating her sealing power--the people of Hyrule united to fight Astor and eventually Calamity Ganon in the Castle Town. Zelda, Link, and their allies subsequently prevailed in defeating them both.