The Gunbad Tunnels consist of three wings, and are suitable for a group of 6 characters between 23rd and 27th level.

Entrance Location
The entrance to the Gunbad Tunnels is in the northwestern portion of the Badlands.

"I's gots betta fings ta do den go ta Gunbad 'n scrap wif a buncha Gobbos! Youse tell Grumlok dat he can come down 'ere and kill me 'imself if 'e wants to. I ain't budgin'!"
-- The final words of Warlord Heava Sixtoes, as overheard by Grumlok

Theme
Night Goblins, considered to be mad by most civilized peoples, infest the tunnels of Mount Gunbad like deranged parasites. In their delving, the Goblins have discovered many amazing funguses, yet they have also uncovered the remnants of the Mourkain Necromancers. In this sense, the Gunbad Tunnels have been made even more dangerous, and woe betide the adventurers who dare to enter the dark place.

Read more about the Warhammer overview here.