The Third Great Migration occurred in 87 ᴇ.ʏ., during the Second Age of Flight. It was the mass exodus of Undertown's entire population (over thirty thousand) towards the Free Glades in the west. They were fleeing from the Dark Maelstrom, triggered when Rook Barkwater accidentally launched Vox's Baby. The migration began in Vox and the rest of it took place during Freeglader.
- Ghosts of Screetown (Led by Felix Lodd)
- Sky Pirates/ Armada of the Dead (Led by Deadbolt Vulpoon)
- Librarians (Led by Fenbrus Lodd)
- Weeg, Wumeru, Molleen, Wuralo
- Flambusia Flodfox and Amberfuce
When the Librarians learnt of Vox Verlix' plan to instigate the Dark Maelstrom and destroy Undertown, they decided to flee to the Free Glades. They set up a trap for both Mother Muleclaw the Second's Shrykes and General Tytugg's goblin army which resulted in the Battle of the Blackwood Bridge. Just before that, the Librarians had started by building a fleet of five giant flat-bottomed barges, carrying all the knowledge of the Great Storm Chamber Library. They left the sewers via a large pipe that fed into the Edgewater River and slowly travelled upriver until they reached the Mire Gates, where they unloaded the barges. The banderbears (Weeg, Wumeru, Molleen and Wularo) carried Amberfuce and Flambusia through Undertown to the Mire Gates; they had taken Vox's place in his bower. The Ghosts of Screetown helped lead all the Undertowners to the Mire Gates.
After reaching the Great Mire Road, the ex-citizens of Undertown were forced to climb down to the Mire because the Dark Maelstrom caused the road to start collapsing. Rook and Felix travelled to the Armada of the Dead to enlist the help of the sky pirates. The pirates and the Ghosts took charge as everyone took shelter for the night. Because the Great Mire Road had collapsed, the scrolls from the library could not be transported anymore. Xanth Filatine proposed distributing the scrolls among the Undertowners which proved a successful strategy.
When the Dark Maelstrom had passed, and the Librarian Knights flew off in three groups of three hundred:
- Varis Lodd led a flight to the Free Glades to get help
- The Professor of Darkness led a flight over the Twilight Woods to monitor any shrykes on that section of the Great Mire Road
- The Professor of Light led a fight to the Eastern Roost to check on the shrykes there
The rest of the group travelled to the northwesternmost corner of the Mire, and onto the northern Edgelands, where they experienced a Sepia Storm. After travelling round the Twilight Woods, they regrouped at the Ironwood Stands, where they met up with the Twilight Woods flight. It was here that the Battle of Lufwood Mount took place.
After the battle, they were able to reach the Free Glades relatively easily, where the citizens could settle in New Undertown.