Necessarius Underground Labyrinth

The Necessarius Underground Labyrinth (ネセサリウスの地下鉄迷宮) refers to the complex arrangement of underground tunnels found in London used and maintained by Necessarius. It was used by Necessarius to test new recruits before becoming official members, notably the Amakusa Christians who passed the actual physical test instead of the "virtual" one that was prepared for them.

By late October of the story's first year, the labyrinth has been decommissioned for being too dangerous, becoming off limits as people kept dying in it, though periodic tests are apparently still carried out.

Principles
The labyrinth are made up of unused tunnels and stations, and is part of the London Underground. This is backed up by the entrances that are connected with the labyrinth such as the subway entrance that Bayloupe and Itsuwa came in, an entrance in the station at (where the Amaskusa Christians mistakenly entered instead at Lambeth due to the Dusk Waiting to Awaken's interference), and the F2 entrance at Lambeth Station for the "virtual test", with it being protected by shutters that need a code to open. In fact, several tunnels connect to that station, though they are all abandoned and the rails have been partially removed.

In spite of this, the tunnels in the labyrinth have fluorescent lights that can be turned off when the test begins. One of the tunnels lead to a station's lost article storage room through a special mechanism allowing for it to be connected to the labyrinth. This is also where items people had left behind or dropped are gathered, and apparently where the results of tests in the labyrinth are kept. Another is the room where the "virtual test" takes place, a perfect cube all in white with each dimension measuring about 20 meters. The room is empty other than pieces of old parchment among other tabletop game components, which allows the "virtual test" to take place.

The tunnels are filled traps, danger zones, as well as routes for the power supply.

Traps
As Necessarius uses the labyrinth, it was filled with countless varieties of traps, both magic and non-magic. This combination allows a greater challenge for magicians who enter the labyrinth as the risk of overlooking something rises.

The labyrinth itself is a large spiritual item, and was made so that the victims that succumbed to the traps would be automatically transported out and any items were dropped there were separated and collected. One trap is the labyrinth being able to magically switch the paths, like a tunnel becoming curved even though the route was supposed to be straight. Other known traps were the transforming corpse wax and the water wheel.

Necessarius Special Admission Test
Before the start members of Dusk Waiting to Awaken had already infiltrated the tunnels and after jamming Freadia Strikers's crystal radio impersonate the Amakusa's in order to carry out their plans. As such, instead of taking the "virtual test" where they are put into an empty room where they simulate traversing through the labyrinth, the Amakusa-Style Remix of Church members instead actually have to traverse the labyrinth and are tasked with making it through there in a time limit in order for them to be able to join the Anglican Church.

The Amakusa later successfully escape the labyrinth and discover that someone has set them up and that Anglican Church would be after them as they are most likely suspect.

British Royal Family Arc
During the operation against New Light, Itsuwa and other members of the Amakusa-Style Remix of Church attempted to apprehend Bayloupe, who fled into the London Underground. After knocking Itsuwa down, Bayloupe bid her farewell and fled the underground station down a tunnel. Believing that she had escaped, Bayloupe was starting to relax herself when she then noticed that the tunnel was curving when it shouldn't. Itsuwa's voice then came over a speaker, revealing to Bayloupe her situation of now being trapped inside the former training grounds. Bayloupe eventually succumbed to the traps in the tunnels while protecting the Skíðblaðnir case.