Shadow Border

The Imaginary Numbers Submersible Shadow Border (虚数潜航艇 シャドウ・ボーダー) is a vehicle which is used by the survivors of Chaldea in Fate/Grand Order: Cosmos in the Lostbelt.

Principles
The Shadow Border utilizes an "artificial Servant", namely a younger back-up of da Vinci, as a navigator and pilot.

Paper Moon
The Shadow Border is equipped with the Imaginary Numbers Observation Device Paper Moon (虚数観測機 ペーパームーン), which is used to facilitate a form of travel devised in the early stages of Chaldea's formation but abandoned due to difficulty and inherent danger.

Taking the opposite approach to rayshifting, where a subject is broken down into Spiritrons and projected to a specific space-time belt, this method involves slipping between the gaps of the world and vanishing from reality, submerging the vessel into Imaginary Number Space (虚数空間).

Usage of the Paper Moon apparently requires authorization from Atlas to operate, and details concerning its operation are contained in the TRI-HERMES records.

History
The Shadow Border was constructed in secret in anticipation of certain enemies that might arise following the restoration of the world in the wake of the Human Order Incineration Incident, concealed within a container normally used for waste disposal. Holmes had also familiarized himself with the operation of the Paper Moon system through looking at the TRI-HERMES records.

During the assault on Chaldea by forces from the Lost Belts, the survivors used the vehicle in conjunction with the waste container ejection system to escape from the facility. Driving through Antarctica, they then witnessed several lights descending from the sky, accompanied by Kirschtaria Wodime's declaration.

Faced with the troops waiting at the coast, Holmes decided to deploy the Paper Moon in order to escape, despite the chance of the system working properly being less than 30%, recognizing it as a necessary step in order to overcome their enemy in the future. The Shadow Border then dove into Imaginary Number Space.