Also known as: 虚数潜航艇, シャドウ・ボーダー
The Imaginary Numbers Submersible Shadow Border is an Armored Vehicle built by Chaldea Security Organization.
It is the primary means of transport utilized by Novum Chaldea to traverse the Lostbelts in Fate/Grand Order: Cosmos in the Lostbelt.
Not to be confused with Vignetting.