The Tower of Kagutsuchi serves as the final dungeon for Shin Megami Tensei III: Nocturne as the time of creation approaches.
After the events of the Diet Building and the summoning of all three Reason gods, the Demi-Fiend will have a final vision of Yuko Takao, who will apologize for her weakness and return to him the empty Yahirono Himorogi, instructing him to use it at the Amala Temple. Once this was done, a vast beam of Magatsuhi was fired from the Temple at the hovering Kagutsuchi, prompting it to extend several stony, column-like "appendages", including one which kept extending until it reached the tip of the Obelisk. It drove the structure into the ground, creating a link between the Vortex World and Kagutsuchi.
Once upon entering, Kagutsuchi will again scan the Demi-Fiend's heart; this act will cause the Old Man and the Lady in Black to leave the Labyrinth of Amala whether the Demi-Fiend has chosen to heed the man's wish to meet at its bottom or not; if the Fifth Kalpa has not been opened before this, it will remain permanently locked.
Regardless of what Kagutsuchi finds in the Demi-Fiend's heart, it will demand the handover of three stones, representatives of Reason, to grant an audience to him. Each of the Reason gods possesses one of the stones; depending on alliances and alignments, the Reason gods may hand the stone over with no fighting.