Justice is massive orbital weapon that is stated to have been the cause of the Great Destruction that forced humanity underground. During the Chrome Path, Murakumo Millenium takes over the weapon and attempts to use it. The player destroys the weapon and thus prevents it from ever being used again.
In actuality, Justice was in fact a shield generator on a grand scale, not a weapon. Before the Great Destruction, it was intended to protect the entire Earth from some greater threat.
Though the exterior is unknown, Justice on the inside, appears as a gargantuan chamber separated into three sections by large energy barriers. The first segment contains a hole leading to a lower level where the energy chargers are located and destroying this will buy a bit more time before it can be fired. At the end is the firing system.