The Upper Three Domains are different from the Lower Six Domains, and the Underworld cities corresponding to the Upper Three Domains are likely located within the Nirvana Realm. This is why the Underworld's Shadow Guard Bureau cannot reach the Upper Three Domains and is unable to obtain any useful intelligence.
"Beneath the iceberg, how could it possibly be the Deputy Director of the Heavenly Sword Bureau? They are Fallen Immortals, and for sword cultivators, the path of the sword ends at the Third-Turn. Are you truly unaware, or are you just pretending?"
Fang Chen smiled faintly. "What is the background of the Awaken-Bright Divine Palace?"
Chapter 493