The Great Cataclysm

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.

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.