Great Cataclysm

"I am Aginah. I sense something is happening in the Golden Land the Seven Wise Men sealed... This must be an omen of the Great Cataclysm foretold by the people of Hylian blood..."

- Aginah

"The Hero's triumph on Cataclysm's Eve wins three symbols of virtue. The Master Sword he retrieves, keeping the Knight's line true."

- Pedestal of the Master Sword

The Great Cataclysm is a prophecy mentioned in A Link to the Past. The conclusion of the Imprisoning War, which took place ages before the events covered in the game, was the sealing of Ganon in the Dark World, with the Triforce in his possession. The Cataclysm refers to a series of actions on Ganon's part, aiming to break the Seal of the Seven Sages and conquer Hyrule. Another part of the prophecy mentions that a hero is going to appear and defeat Ganon.

Ganon's Plan
Ganon used a wizard named Agahnim as a puppet in order to achieve his aim. First, Agahnim was able to gain the trust of the King by dealing with a series of disasters that occurred earlier in Hyrule. Although the origin of the disasters is not known, it is very possible that Agahnim himself was the cause. He proceeded by removing the King and taking his place on the throne. A spell was cast on the soldiers, brainwashing them, and ultimately giving him full control over the castle. The next phase was to abduct seven Maidens, descendants of the Seven Sages, in order to transport them to the Dark World. By sacrificing them, Ganon would finally be able to break the Seal.

On Cataclysm's Eve
Link is summoned in the beginning of the game, telepathically by Princess Zelda, who is imprisoned in the castle. She is the only one of the Maidens that is still left in Hyrule. The rest of them are already transported by Agahnim in the Dark World. After leaving her in the safety of the Sanctuary, Link meets Sahasrahla, who instructs him to retrieve three pendants, scattered across different parts of Hyrule. Only then he will be able to retrieve the Master Sword, which was specially crafted to be effective, even against one that would have the Triforce in his possession.

Link discovers the Master Sword in the Lost Woods. An inscription was engraved on its pedestal: "The Hero's triumph on Cataclysm's Eve wins three symbols of virtue. The Master Sword he retrieves, keeping the Knight's line true." As it proved, the hero that the inscription was referring, was Link, probably the last descendant in the bloodline of the Knights of Hyrule, who fought along the Sages during the Imprisoning War.

The point where Link pulled the Master Sword out of its pedestal was critical, as Ganon was almost successful in his plan to release the seal. Agahnim had already found Zelda and was about to send her to the Dark World. When Link returned to the castle to face Agahnim, it was too late for Zelda. Nevertheless, Link defeated Agahnim and proceeded to rescue the Maidens from their imprisonment in the Dark World. The maiden Link rescues in the Skull Woods, makes a reference to another part of the prophecy, which states that whenever an evil person seizes the Triforce (as Ganon did during the Imprisoning War), a hero is destined to rise up and vanquish the evil one. Furthermore, the prophecy states that the hero must be of the bloodline of the Knights of Hyrule. This seems to indicate that the Cataclysm ends with the battle between the Hero armed with the Master Sword and the evil person holding the Triforce.

Book of Mudora
Another reference to the prophecy is made when Link meets Aginah, in the Desert of Mystery. He explains that according to the prophecy, Link, if indeed is the hero that will defeat Ganon, he has to use the book of Mudora, in order for him to be able to read the language of the Hylians.

As a Recurring Prophecy
Due to the similarities between the events of Ocarina of Time and A Link to the Past, some have theorized that the prophecy of the Great Cataclysm could refer to a series of similar events that happened in different periods in the history of Hyrule, each time a hero showed up against the evil one who possessed the Triforce. Thus, the prophecy would apply to the Links of Ocarina of Time, Twilight Princess, The Wind Waker, and A Link to the Past, as all of them defeated a Triforce-wielding Ganon with the Master Sword and prevented a conquest of Hyrule. The specific details regarding the Link of A Link to the Past (The Pendants, the bloodline, the Book of Mudora) could have been simply more specific references to the prophesied hero from A Link to the Past. A similar legend is told on the back of the box for The Wind Waker, referring to a prophecy that states that whenever evil arises, a hero named Link will defeat it. The Great Cataclysm could be a similar prophecy.

Trivia

 * In the instruction manual for Link's Awakening, another reference to the prophecy is made. Link "fulfilled the Hyrulean prophecy of the Legendary Hero and destroyed the evil tyrant Ganon."