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 is an event from The Legend of Zelda: Breath of the Wild. This event serves as the main historical context of the game. It corresponds to the disaster caused by Calamity Ganon throughout Hyrule that occurred on Princess Zelda's 17th birthday one hundred years prior to event of the game. After Link awakens at the Shrine of Resurrection at the beginning of Breath of the Wild, he discovers much of Hyrule ruined and devastated by Ganon's forces. However, thanks to Princess Zelda sealing herself and Calamity Ganon away in Hyrule Castle the rest of Hyrule managed to survive and somewhat rebuild.

Calamity Ganon first emerged in the distant past, ten thousand years before the events of Breath of the Wild. Ganon transformed into a monstrous creature of pure Malice known as Calamity Ganon. He ravaged Hyrule before being sealed away by the chosen Hero wielding the blade that seals darkness, and by the princess with divine powers inherited from the Goddess. Assisting the hero and the princess were the Guardians and four massive mechanical animal constructs called the Divine Beasts. The Ancient Sheikah tribe developed these machines with their unique technology, to help the Royal Family of Hyrule. Ganon.

Ten thousand years later, during the reign of Rhoam Bosphoramus Hyrule, it was foretold that the return of Calamity Ganon was inevitable. The prophecy of Ganon's imminent return forced the Royal Family and Sheikah still loyal to the kingdom to excavate the ancient technology in preparation to confront Calamity Ganon as their ancestors had done 10,000 years prior. King Rhoam decided to act in accordance to their ancestors' plan and excavated the relics, including the Guardians and the four Divine Beasts. They recovered the mechanical army and were successfully able to reactivate and use them. The teenage Zelda assisted them in their research due to her interest in science as she still had trouble awaking her powers. King Rhoam then formed a group of exceptionally skilled warriors referred to as Champions lead by his daughter Princess Zelda. He requested each race of Hyrule choose a Champion, and four elite warriors were selected from across the kingdom by Zelda, Mipha the Zora Champion and crown Princess, Daruk the Goron Champion, Revali the Rito Champion, and Urbosa the Gerudo Champion and Chieftain. They were tasked with the duty of piloting the Divine Beasts to assist the Hylian Champion, the knight Link. The final key to defeating Calamity Ganon was the divine sealing powers of the royal family's bloodline, and the recent death of Hyrule's Queen saw this immense duty fall to her only surviving daughter who lost not only a mother but a mentor. As she was now sole bearer of this birthright which she inherited from her grandmother, the King ordered Zelda to focus on awakening her divine sealing powers to the exclusion of all else.

Unfortunately, having learned from his previous defeat, Calamity Ganon created four phantom embodiments of himself representing the elements of fire, water, thunder, and wind, each tasked with corrupting and taking control the Divine Beasts, and killing four of the Champions and imprisoning their spirits in the process. Succeeding in taking control of the Divine Beasts, Calamity Ganon proceeded to take control of the Guardians with his Malice to use against the people of Hyrule. He then destroyed Hyrule Castle Town and devastated much of Hyrule, slaughtering countless innocents. Among the chaos, King Rhoam was killed and the remaining soldiers and Knights of Hyrule who survived the attack on the castle were helpless against the Guardian onslaught and were forced to retreat to the Akkala Citadel in Akkala. The final remnants of the Hyrulean Army took their final stand in Akkala Citadel, the kingdom's largest stronghold, which was all but destroyed in the historic battle, leaving the once great structure in ruins.

Zora's Domain was attacked at this time, though the bottleneck that is Zora River allowed the Zora Knights to hold off the assault very effectively and their army survived to continue defending Zora's Domain. However the Zora people greatly mourned Mipha's death and many Zora Knights and adults blamed Link and the Hylians for the Great Calamity. However King Dorephan though regretting having allowed his daughter to fight understood she would not forgive him for holding Link responsible and realized they had all underestimated Ganon's cunning and power. Though he was very young at the time, Mipha's younger brother Prince Sidon would come to share his father's views in the years to come with the Zora out living most of their non-Zora allies due to Zora having longer lifespans and aging at a different rate.

Link and Zelda, meanwhile, had been unable to help in Central Hyrule, and retreated south to Fort Hateno where they made their desperate last stand against an army of Guardians. Though Link wielded the True Master Sword which reacted to Ganon's corruption within the Guardians with skill and personally destroyed countless Guardians himself, they were simply too numerous and he was eventually overcome.

It was here, faced with the impending death of a loved one, that Zelda's powers were truly awakened, allowing her to shut down the Guardians with a blinding ray of light by purging them of Ganon's influence. The Guardians had been stopped from destroying Link, but despite being saved by Zelda he still succumbed to his injuries and collapsed to the ground, seemingly dying in Zelda's arms.

Heartbroken, Zelda wept in despair. As if responding to her cries, the Master Sword activated on its own and spoke to Zelda. It revealed to her that Link was not yet dead and could still be saved reminding Zelda of the Shrine of Resurrection an ancient Sheikah medical facility her research team had discovered. Wasting no time, Zelda ordered her Sheikah allies, Impa, Purah, and Robbie, to deliver Link to the Shrine of Resurrection upon the Great Plateau, and to place him in the experimental Slumber of Restoration. Zelda then returned the battle-damaged Master Sword to its pedestal in front of the Great Deku Tree, making to the blade the promise that Link would one day return for it. In a final sacrifice, Zelda marched alone on Hyrule Castle and used her newly awoken powers to seal both Ganon and herself inside, ending the Great Calamity.

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.