Fairy Meng Shang was clever as ice and snow. From Ling Tian's words and actions, she could easily tell that he wasn't planning to have her, Feng Lingzi, and the others join him in facing the tribulation. After hearing what she said, Xiao Shi realized it too. He grew agitated, because he thought Ling Tian might "abandon" him as well and face the tribulation alone. That was the last thing he wanted to see.
It was easy for Fairy Meng Shang to figure out why Ling Tian planned to do this. Since he wanted to train a final disciple to protect the cultivators left behind in the God Realm, it was obvious that those who would be left behind were none other than them.
It made sense. Fairy Meng Shang could readily see that Ling Tian wasn't entirely confident he could take everyone out of the universe. If so, he certainly wouldn't let them face the tribulation alongside him. Once they chose to break their seals and face the tribulation, there was only one path besides success: death. Cultivators who failed to break free of the Lord of the Universe's restraints would perish for lack of life force, or even die directly under the tribulation lightning.
That was right. If Ling Tian couldn't take them out of the universe, they would likely be struck dead by the tribulation lightning. Few of them would even be left behind as hostages for the Lord of the Universe.
And what if some of them were taken hostage? Since they would have broken their seals to face the tribulation, and each cultivator could only be sealed once, they wouldn't last long after breaking free of the seal. In other words, any cultivator who failed to escape the Lord of the Universe's restraints would inevitably