Nemesis Horizon

The Nemesis Horizon ( 天の岩戸ネメシスホライゾン ) is a cauldron that floats in the sky. Summoning it and opening its gate under certain conditions with the Successor of the Azure allows the Sankishin - Amaterasu Unit to be summoned into the world.

Information
Izanami, Relius Clover, Hazama, and Yuuki Terumi desired to enact Doomsday, but in order to do so, the Nemesis Horizon needed to be brought into the sky above the 5th Hierarchical City of Ibukido. After sacrificing countless souls to the Monolith in the middle of the ruined city, Terumi had Nu-13 coax the Nemesis Horizon into appearing.

As Nu had recently synchronized with Noel Vermillion, both Amaterasu and the Nemesis Horizon recognized Nu as the Successor of the Azure, and it descended into the world, opening its gates and allowing Amaterasu into the real world. The Nemesis Horizon remained open as the Embryo was created, but its whereabouts afterwards remains unknown.

Nemesis Horizon
Something Hazama was trying to open in the story of 'Chrono Phantasma.'

''It's a gate which allows the descent of the 'Master Unit, Amaterasu', and only the Successor of the Azure can open it. The gate was opened by Nu after she absorbed Noel, becoming a false Successor of the Azure, causing the 'Master Unit, Amaterasu' to descend into the world.''

Trivia

 * Nemesis Horizon is written with the characters for the Ama-no-Iwato, the legendary cave in the heavens that the goddess Amaterasu once hid inside.