Ch. 421The Shinobi World's Ultimate Final Boss—Naruto, Uchiha Sasuke
Naruto and his allies completed a vast eighteen-floor Underground City, turning it into both a shelter and a monster-filled training dungeon. As Tobirama and Hiruzen Sarutobi followed Naruto through its eerie passageways, Tobirama questioned whether Naruto could be trusted.
- Naruto's Shadow Clones and his allies excavated an Underground City three thousand meters beneath the Shinobi Alliance's territory and connected it to several newly joined lands.
- Naruto divided the city into eighteen floors, reserving the first as a living area and filling the lower levels with increasingly dangerous monsters, traps, and treasure chests.
- Naruto left the eighteenth floor empty except for a Golden Apple marked with the Flying Thunder God technique, which would summon Naruto and Sasuke as the final bosses.
- Tobirama recoiled at the dungeon-like entrance and complained that the city was meant to be a shelter, not a prison.
- Tobirama asked Hiruzen Sarutobi whether Naruto was reliable, and Hiruzen reassured him that Naruto would not deliberately endanger everyone.