"In the year 1994, from out of space comes a runaway planet, hurtling between the Earth and the Moon, unleashing Cosmic destruction."
The Great Cataclysm was such a catastrophic event that it destroyed a large percentage of Earth's population and demolished much of human civilization. The aftermath of this event led to several other races (such as Moks and Carocs) coming to control large sections of the planet.
how the Earth changed[]
Geography[]
While many parts of the world survived, it's clear that the cataclysm's immediate effect was enormous. The Earth isn't cracked to pieces, but for a time had incredible short-term instability.
Magic[]
As Mindok points out, the Cataclysm changed the world and somehow awakened the potential to use magic. But this wasn't just in himself.
Then, as Khromm demonstrated with the Moon Dial, the Cataclysm didn't change the physical laws of the universe. Khromm's sorcery worked just fine before the Cataclysm.
So the real change was whether there were people on Earth who could wield magic.
the Moon[]
Earth's moon is seen being cracked apart by the shock of the runaway planet passing so close to it, and in all scenes where the moon is shown there are visible cracks. But this varies from episode to episode, and might represent some sort of lingering instability in the moon.
Remnants[]
There are several objects and a few people that were left over from before the Cataclysm.
People[]
The Wizard Mindok was apparently an ordinary human before the cataclysm.
The "Ice People" that Mindok sought to recruit as servants were NASA scientists before the Cataclysm.
The people of the City of Thieves aren't individuals from before the Cataclysm, but a society that has persisted since before the Cataclysm.
Places[]
Many episodes show ruins of cities and places that existed before the Cataclysm, Alamo and Manhat are two examples. The City of Thieves survived the Cataclysm inside their containment field.
Technology[]
There were several technological advances shown to have been made before the Cataclysm that do not exist in the real world. Examples include the Guardian machine that protected the Alamo and various mutated monsters that were said to have been created as a result of genetic research prior to the Cataclysm, such as the Sleeping Demon.