The Dark Maelstrom.

The Dark Maelstrom, occasionally known as The Black Maelstrom, was an enormous raging storm, created by Vox Verlix as revenge on all who betrayed him. The storm destroyed Undertown, the Tower of Night, the Sanctaphrax Forest, and the Great Mire Road, in addition to killing the Guardians of Night and forcing the Undertowners, Librarians, Ghosts of Screetown, and Mire sky pirates to flee to the Deepwoods. This exodus was known as the Third Great Migration.

This Storm was created by a device invented by Vox Verlix, a sphere full of phraxdust and crushed bloodoak acorns which would explode on contact with nature. This invention, named ‘Vox's Baby’ was launched into the centre of the hot, heavy cloud obscuring the sky above Undertown, triggering the Dark Maelstrom. The storm caused extremely heavy rain, which flooded and therefore destroyed Undertown and its surroundings.