The Immortal World is vast.
Yet the Immortal World is also small.
Because the vast Immortal World has long been occupied by various forces: the Heavenly Court, the Three Great Saints, the Dragon Clan, the Qilin Clan, the Phoenix Clan, the Demon Clan, and many other extraterrestrial races. Every place already has an owner.
This includes the Hundred River Immortal Domain, which on the surface is an area under the dominion of the Heavenly Court.
Therefore, when the Divine Monarch Mansion summons, if it's an average Golden Immortal force, even if the old Flood Dragon breaks through to the Taiyin Immortal Realm, it would be impossible to refuse.
Under normal circumstances, if the Divine Monarch Mansion summons, it wouldn't be a problem for Su Yu to go there, to meet the Immortal World's Heavenly Court Divine Monarch and see what kind of Great Divine Power he possesses, and what kind of immortal scenery the Divine Monarch Mansion offers.