- Shin Megami Tensei III: Nocturne: Antagonists
The Assembly of Nihilo is the group created by Hikawa in the Vortex World to serve his vision of a world of silence. It is a group that places intellect above strength and thus considers the rival Mantra Army as a mere annoyance in the great scheme of things: while Mantra plots to rule above the chaos of the reconstructed Tokyo, the Assembly's plan is to rewrite reality itself through the power of Kagutsuchi. To this end, they siphon the free-floating Magatsuhi from the Vortex World with a complex system to allow for the summoning of Nihilo's Demonic Sponsor - Ahriman, god of the void.
The Assembly's Shiodome base is situated near Ginza, though it remains inaccessible until the Demi-Fiend has successfully contacted Gozu-Tennoh, unwittingly prompting the invasion by the Mantra Army. Following this, the Assembly is attacked and the core unit of their Magatsuhi-absorbing machine apparently destroyed. However, the Demi-Fiend will find Hijiri in the ruined core unit, where he will learn the system was not damaged and in fact the Mantra has been fooled into attacking a dummy base.
Further into the Assembly's building, the True Core unit lies, protected by four special keys. When the Demi-Fiend successfully penetrates the defensive lines until reaching the core unit, searching for Yuko Takao, instead he will find Hikawa, who commends him for his efforts but decries them as useless. Following this, he summons Ose and activates his special weapon, the Nightmare System, instantly weakening the Mantra Army and leaving the Assembly as the, at the time, sole power structure left in post-Conception Tokyo.
Following the fall of Mantra, the Demi-Fiend set out to find his teacher; Hijiri located the Nightmare System in the Obelisk, which was said to be her most recent location. Disabling the edifice's security traps and killing its wardens, the Moirae Sisters, he released Yuko from the System, shutting it down. Later, he found Hikawa at the ruins of the Mantra Army HQ; he explained he had realized who the Demi-fiend was, and explained his vision of the world to him.
He also explained why he had made no visible effort to stop him - he already had the Magatsuhi he needed, just from the raid at Mantra, he just wanted to confirm his theories and see if he was, indeed, interested in following his plans for the creation of a world of stillness. Hikawa also warned the Demi-Fiend about the possibility of the reemergence of the spirit of Gozu-Tennoh.
In his final act to race against the established Reasons of Musubi and Yosuga, the Assembly took over the Diet Building, securing it with the aid of Surt, Mithra, Mada and Mot, and twisting it with false halls and dummy doors. After the destruction of the four Nihilo demons, the Demi-Fiend ascended to the sixth floor of the building and either confronted Hikawa, in which case he was forced to battle Samael, or merely met with him (if he himself was a follower of Shijima) and witnessed the arrival of Ahriman and the death of Yuko.