The Great Cataclysm was an event that involved the destruction of the Dark Tower and a lot of the surrounding land, as well as causing a magical disease violently twisting the appearance and personality of people, creatures and even minions as well of the land,turning it into a black wasteland covered by Magical Slime.it happened shortly after the minion exodus started
Relatively early in Overlord II, while exploring the Wasteland that was once the area around the Dark Tower, Gnarl comes to the realization that the Great Cataclysm was the result of the Tower Heart exploding. Near the end of Overlord II it is revealed that Florian Greenheart tried to steal the Tower Heart's magic, but caused it to explode, resulting in the Great Cataclysm and the Magical Plague.
Theory About What could have happened
The great cataclysm happened because of Florian not having any magic. But Florian wouldn't know where the Dark Tower is and Rose seemingly is wanting to place revenge on the Third overlord for dumping her for Velvet. seen as she led the Second Overlord to the tower,(Even though the Second overlord already knew the location),For revenge.She might have went to the heavens peak abyse and found the Forgotten god and he set a trap for the third overlord. When the third overlord went into the Infernal abyss she expected him not to come out and killed by the forgotten god,but he survived(although blocked) and left his son to be his successor. Meanwhile After the exodus Florian had caused it to go unstable (again) and cause the tower heart to explode after the Minion Exodus. It is possible that Rose and Florian suffered Minor symptoms of the Magical plague,since they were presumely near or inside the tower when the Tower Heart exploded. Rose changing in personality while Florian pretty much has developed minor magic powers,like being able to walk in the ooze of the wasteland without being infected or harmed