The Undercity

The Undercity is a strange and sprawling environment. The pathways twist and warp into arrangements that don't resemble the city they used to belong to. Maps from before the land was swallowed by the darkness are useless. Divers must instead rely on their own intuition and the guidance of Navigators, friendly voices on the outside, communicating through navigation equipment attached to the Dive Gate. The landscape of the undercity changes from visit to visit, so Divers need navigators to help find entrances and exits, and to find hidden places that the undercity doesn't want to show them.

Scavenging

Divers enter the undercity to forage for supplies. While its make-up is chaotic, the Undercity still contains many resources that can be of use to the citizens of Diamond City. Metal scrap, simple tools, even some appliances have been refurbished after being found in the undercity. By far the most important resource, however, can only be found within the undercity. It's a mysterious glowing ore that can be found organically running through buildings and imposing itself on the decaying structures of the undercity. Most call this material "Mithril" after the fantasy material made famous by fantasy stories of old, but Numeral 5's internal documents refer to it as "Tenebrite" or "Umbral Ore." This material is what makes much of the Diver's gear possible. 

Investigation

The next most important reason for Divers to be in the Undercity is for investigation. This generally means finding any information on the distortion itself, or the origin of husks. Sometimes scientists accompany the Divers on their missions to gather samples or study something directly. Any investigation mission is well-planned in advance and organized. The goal here is generally to protect the scientist and keep the plan from going off the rails.

Rescue

Strange things happen on level 4 of the city sometimes. People occasionally go missing near the dive gates or on diving missions. In situations like this, rescue missions can sometimes be launched. Finding missing people in the Undercity is difficult, even for navigators, and rescue teams are often monitored by high-level personnel to make sure they don't delve too deep and risk losing more people to the darkness. Only about 40% of rescue missions are successful, and even then, people who spend extended amounts of time in the undercity can find their bodies and minds changed irrevocably.

Sport

No operation is perfect, and while the powers that be try their best to keep Divers in line, Divers occasionally slip between the protocols and get themselves an objective-free romp through the undercity. It is dangerous, and sometimes they do not return, but some people would rather tempt fate than live quiet lives.

Obstacles and Challenges

Husks

Husks are masked monstrosities that roam the Undercity and occasionally claw their way up into Level 4. Most appear to be mindless, but more powerful ones have been known to act intelligently. All husks wear "masks" that resemble contorted and bizarre human faces. Destroying these masks cause the husks to disintegrate into "husk dust." There is no bargaining with husks, as they attack Divers on sight and seem to be able to sense when humans are nearby. Patroling divers in Level 4 often catch renegade husks making their way towards the few community blocks that still house civilians on that level. The Dauntless brand, who specialize in husk combat, moved their headquarters to level 4 in order to provide a "bigger target" for husks. This has largely been successful in drawing errant husks away from smaller urban areas.

While rare, there have been sightings of humanoid husks. They are referred to as "Harlequins" because their masks are simpler and less descriptive than their weaker counterparts, like those used in stage plays. The few recorded sightings of these Harlequins almost always result in the death of accomplished divers, and they are considered to be extremely dangerous. Divers who encounter this type of husk are advised to flee at any cost.

Shadows (Perfect Draw AU only; replaces Husks)

The most common threat that Duelists will face in the Undercity are Shadows. Shadows can take many forms. They generally appear as some kind of humanoid figure, seemingly composed of some kind of matter present in the area. Shadows use magical crystals which grow in the under city in order to conjure creatures, objects, and effects that bear striking similarities to the more refined card effects generated by Duel tech. Shadows tend to be hostile, and will attack a Duelist on sight. Once they've broken a Duelist's shield, they will not hesitate to try to drag that Duelist into the shadows, from which they might never return.

Of particular note are stronger, more sentient shadows that are generally referred to as Theurges. Theurges tend to have strategies and appear to understand Duelist intentions, sometimes controlling lesser Shadows in order to manipulate or ambush unwary Duelists. There are rumors of Theurges that can imitate and understand human speech.

Challenge Cards (Perfect Draw AU only)

Sometimes Duelists will encounter card-like runes while exploring the Undercity. These are referred to as Challenge Cards. Challenge Cards present challenges which Duelists must overcome in order to progress. Sometimes they stand in the way of the objective and other times they are encountered by happenstance, offering an optional side objective which can be completed. Challenge Cards are of great value to Numeral 5. The special crystals that can be obtained from them have been used to create new advancements in duel-tech, and the Duelists who secure these tend to reap the rewards of these advancements.