The Great Calamity is the terminology used to define a series of events from the The Legend of Zelda series that serves as the main historical context of The Legend of Zelda: Breath of the Wild. It corresponds to the disaster caused by Calamity Ganon throughout the kingdom of Hyrule that occurred one hundred years ago before the beginning of Breath of the Wild, causing the destruction of many important location such as the Temple of Time, the Akkala Citadel or even Hyrule Castle
Ganon's previous attack
Ganon now known under the name Calamity Ganon last emerged Ten thousand years before the events of Breath of the Wild, following the cycle of reincarnation dictated by Demise's curse. He ravaged Hyrule before being sealed away by the chosen Hero wielding the blade that seals darkness, and by the princess with divine powers. It was defeated thanks to an army of autonomous mechanical 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.
The Sheikah exile
According to the Sheikah Cado, his tribe had long been heralded as people of great wisdom since their ancient technology became the key in sealing Calamity Ganon ten thousands years ago. Their technology was at first praised as the power of the gods. However, the people of Hyrule began to fear it as a threat to the kingdom. As a result, the Sheikah became outcasts and was forced into exile. Although most Sheikah chose to cast off their technology (which became a forgoten part of Hyrule history) and live simple lives, others fostered hatred towards the kingdom of Hyrule, leading them to swear allegiance to Ganon. This resulted in the creation of the Yiga Clan, who dedicate themselves to eliminating all who stand against Ganon.
Anticipating the Great Calamity
Reactivating of the Sheikah relics
Ten thousand years later, and one century before Link's awakening inside the Shrine of Resurrection on the Great Plateau, it was foretold by the fortune teller that the return of Calamity Ganon was inevitable. This prophecy drove King Rhoam Bosphoramus Hyrule to create an archaeology team to search for the fabled Ancient Sheikah's technologically-advanced relics. King Rhoam decided to act in accordance to their ancestors' plan and excavated the relics, including the Guardians and the four Divine Beasts. The Sheikah still loyal to the Royal Family including Impa, Purah, and Robbie assisted them in studying this ancient technology. They recovered the mechanical army, and were successfully able to reactivate and use them. Zelda assisted them in their research due to her interest in science as she had trouble awaking her powers. Many believed them to be simply a legend but their discovery made this legend a reality for them.
The choosen hero and the Champions
Realizing this, the King selected the hero chosen by the Master Sword, a young recruit of the royal army named Link. He was appointed as the personal knight of his daughter, Princess Zelda. King Rhoam then formed a group of exceptionally skilled warriors referred to as Champions in order to pilot the Divine Beast. He requested each race of Hyrule choose a Champion, and four elite warriors were selected from across the kingdom, Princess Mipha the Zora Champion, Daruk the Goron Champion, Revali the Rito Champion and Urbosa the Gerudo Champion.
The Divine Power
The final key to defeating Calamity Ganon was the Divine Power of the royal family's bloodline, and the recent death of Hyrule's Queen saw this immense duty fall to her only surviving daughter. As she was now sole bearer of their birthright, the King charged Zelda to focus on mastering her control over the Divine Power to the exclusion of all else, and duly appointed Link as her personal knight and bodyguard to ensure she stayed safe.
While the other Champions piloted their race's respective Divine Beast, Zelda was sent on a pilgrimage under Link's protection, praying to statues of the goddess Hylia for the power to defeat Calamity Ganon. During this preparation period, the Champions bonded with each other, with Zelda growing especially fond of Link once she realized their similarities as people burdened with tremendous legacies to live up to.
The Great Calamity
However, Zelda was unable to unlock her power in time, and all of this preparation proved to be in vain. Calamity Ganon was prepared for Hyrule's attempt to repeat history and proved to have learned from its previous defeat by using its near limitless dark power. Recalling the threat these enemies posed, Ganon reappeared and created four phantom embodiments of himself known as Blights representing the elements of fire, water, thunder, and wind, tasked with corrupting and taking control the Divine Beasts, and killing four of the Champions in the process.
Calamity Ganon used his own dark power to take control of the Guardians against the people of Hyrule, destroying Hyrule Castle Town and the countless settlements across Hyrule. King Rhoam was killed and the remaining soldiers and Knights of Hyrule who survived were helpless against the Guardian onslaught. Link was confronted by an army of Guardians at Fort Hateno, valiantly defended both the Fort and Princess Zelda wielded the Master Sword. Despite destroying countless guardians, his strength and skill were not sufficient to overcome their numbers and he was gravely injured.
With the heroes' defenses crumbling, and even Link overwhelmed by the renewed assault, Zelda finally unlocked her power and saved Link from the fatal blow. She requested her Sheikah allies Impa and Robbie, to send him in the Shrine of Resurrection on the Great Plateau, where he would recover in a deep sleep for the next one hundred years. Zelda then returned the battle-damaged Master Sword to its pedestal in front of the Great Deku Tree. As she revealed to the Great Deku Tree, the Master Sword told Zelda that her destiny was not finish. Zelda eventually used her Divine Power to seal Ganon and herself within Hyrule Castle.
After the fall of Hyrule during the Great Calamity, and the seal of Calamity Ganon in Hyrule Castle by Princess Zelda, the surviving people of Hyrule recovered and settlements like Hateno Village where rebuilt thus beggining the Age of Burning Fields. Due to King Rhoam's death and Zelda's disappearance, the kingdom lacked any proper leader during the Age of burning field, however, Impa was by her status in the fallen kingdom the highest authority left and any official writ must be approved by her.