The countless floors under the Citadel of the Levinkan would make up one of the most nefarious and dangerous dungeons in the realm.
Floors
Floor 2: The LevinZoo
The Levinkan's personal menagerie of unique and fascinating creatures was only two level underground, and housed creatures such as the Uniduck, the Minitaurs, Jet Propelled Piercers, and killer Dolphins.
Floor 5: The White Plume Geysers
Five floors under the citadel housed a massive chamber that was originally built to harness hot steam from underground, but became a dungeon full of various invaders such as Kelpies (who killed Slackjaw). The Levinkan tried on various occasions to send adventuring parties under ground to tackle the threats, but ultimately conceeded to allowing them to live under his citadel as long as they did not disturb the peace on the surface.
Snarla and Burket
Snarla and Burket were a group of werewolves who were trapped in the UnderCitadel, as any attempts to rise through the chambers back to the surface led to confrontation with the Levinkan's forces. Instead, the two decided to reside in the caves and feast on any intruders, such as Hamjaw and Willow (the latter of which survived and contracted the disease).
Ctenmiir
Cternmiir was a vampire who also lived in the UnderCitadel, but chose to reside here by choice, as the lack of sunlight and daytime means he can stay active through all hours of the day, though he did occasionally rest when truly in need of relaxation.
Behind the Screen
As the UnderCitadel is meant to be an endlessly deep dungeon, many published dungeon elements have been used in its play, most notably elements from 1979's White Plume Mountain.
