Even with the strength of Domain Level, if one lingers in this world for too long, it's likewise possible to lose one's life to the various dangers present. Only if members of those otherworldly organizations do not make contact with any living beings from the Main Plane and also do not stay for too long can they manage to leave the Main Plane alive.
Because of the unique predicament of those organizations, Zhou Tian had initially not considered the possibility that the two Monsters would return to trouble him. Moreover, it was exactly because members from organizations outside of the Main Plane often faced various troubles that Zhou Tian felt worried at this moment.