The Great Rift is manifested by the power of the magic green emerald of Khufu. When the jewel is worn, it creates great anger inside its user, poisoning their heart and filling it with evil. The more fighting that the wearer engages in, the more power that is transferred into the Great Rift, making it more powerful and solid over time.
The Green Emerald of Khufu somehow ended up in Mesmaria over the years, where the beast was eventually summoned and reduced the once glorious city to dust.
The Great Rift was summoned once more when Queen Deluca drew plans to take over the Seven Deserts. However, her dear brothers did not like the idea of her becoming the most powerful ruler of Agrabah. The siblings started arguing and ended up using their magic against each other. This caused the mysterious Great Rift to appear, revealing it to not be a natural disaster- it takes the true form of a massive, transparent, green monster with red eyes, curved horns, and fangs of a saber-toothed cat. Aladdin discovered this and tried to tell the family that the monster fed on anger from the Mesmarians, but they don’t listen and continue fighting each other, destroying the palace. Aladdin seeks the help of King Zahbar, the only Mesmarian not affected by the emerald's power. He tells Aladdin that the jewels have great magic, as well as great anger, causing the Great Rift to be brought into existence. It was this very magic that corrupted Deluca and her brothers. The gang rushes back as the demon gets closer and closer to the palace.
Eventually, Aladdin, Abu, Iago, Carpet, and Genie work together to get the emerald away from Deluca, and Aladdin uses a huge hammer from Genie to smash it to bits. The Great Rift, who is about to break into the central tower of the palace, roars in pain and flickers several times before finally vanishing into thin air. Peace is restored and the Mesmarian family is free of their curse.