Great Cataclysm

On Cataclysm's Eve
The Great Cataclsym was the moment when the evil Gerudo King of Thieves, Ganondorf, entered the Sacred Realm and took the Triforce of Power for his own. After the Hero of Time won "three symbols of virtue" (the Kokiri's Emerald, the Goron's Ruby, and the Zora's Sapphire), he used them alongside the Ocarina of Time to open the Realm's enchanted door, the Door of Time. With the door opened, the Hero of Time claimed the Master Sword from the Pedestal of Time and was subsequently sealed within the Temple of Light for seven years to fully become the Hero of Time. Meanwhile, Ganondorf followed the Hero of Time through the Door of Time and laid his tainted hands on the Triforce, unleashing what history would come to call the "Great Cataclysm." This would be followed by the events of the Imprisoning War, which some believe to be chronicled in Ocarina of Time.

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

- Pedestal of Time, A Link to the Past

"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

"Do you know the prophecy of the Great Cataclysm? This is the way I heard it... If a person who has an evil heart gets the Triforce, a Hero is destined to appear... ...and he alone must face the person who began the Great Cataclysm. If the evil one destroys the Hero, nothing can save the world from his wicked reign. Only a person of the Knights of Hyrule, who protected the royalty of Hylia, can become the Hero...You are of their blood-line, aren't you? Then you must rescue Zelda without fail."

- Maiden in the Skull Dungeon

Theories Surrounding the Great Cataclsym
According to A Link to the Past, the Great Cataclysm occurred when the Gerudo King of Thieves, Ganondorf, entered the Sacred Realm (or Golden Land) and seized the sacred Triforce for himself. The Great Cataclysm then led to a prolonged and bloody conflict known as the Imprisoning War that resulted in Ganon's (as Ganondorf came to be called) "imprisonment" inside the Sacred Realm, which became the Dark World due to his vile influence. These events were chronicled in Ocarina of Time, when the Hero of Time pulled the Master Sword from the Pedestal of Time and opened the gateway to the Sacred Realm. In doing so, he was sealed in the Sacred Realm for seven years, and Ganondorf used the gateway he opened to enter the Sacred Realm, claiming the Triforce with blood-stained hands. The "three symbols of virtue" the Hero supposedly won were the three Spiritual Stones (which possibly became the Pendants of Virtue centuries later). The Hero's heritage "keeping the Knights' line true" seems to indicate that the Hero of Time, the Hero at the time of the Great Cataclysm, was a descendant of the Knights of Hyrule. If this is so, it would lend credence to the possibility that the Link from A Link to the Past is a descendant of the Link from Ocarina of Time since the one from A Link to the Past was noted as being the last of the bloodline of the Knights of Hyrule.