Monolith

A Monolith, also known as a Tablet or Black Pillar, is a massive structure used to absorb souls. One was constructed on top of the Novus Orbis Librarium branch in the 13th Hierarchical City of Kagutsuchi, and another in the middle of the 5th Hierarchical City of Ibukido.

Information
Monoliths are colossal black towers that can come to life with numerous blue veins and red runes appearing across its body. Their primary purpose is to absorb souls which can then be used for numerous purposes, such as fuel for the smelting of a Boundary Interface Prime Field Device. This was demonstrated as Noel Vermillion was smelted into Mu-12 using the Monolith's accumulated souls. Kagutsuchi's Monolith still remains in its location, and is directly connected to the cauldron in the lowest level of the NOL branch.

Izanami later had a monolith erected in the Ikaruga Region. Despite being in Ibukido, it was visible across the entire region, being far bigger than the one in Kagutsuchi. To enact Doomsday, this Monolith was activated before Nu-13 summoned both the Nemesis Horizon and the Amaterasu Unit into the world. The activation of the device killed countless Ikarugans. The base of this Monolith was destroyed by the combined efforts of Jin Kisaragi and Hakumen shortly after its activation.

Monolith
''Another name for Tablets. In the story of 'Continuum Shift,' Hazama and Relius used a Tablet and a Cocoon to smelt Mu. In the story of 'Chrono Phantasma,' the Imperator constructed one in Ikaruga.''

Black Pillar
''A towering pillar standing in the middle of Ikaruga. It's so tall that it can be seen from anywhere in Ikaruga.''

It is actually a 'colossal Monolith', similar to the Monolith built by Hazama at the top of the 13th Hierarchical City 'Kagutsuchi.' Its base is located in the ruins of 'Ibukido,' and it's connected to the Cauldron underground.