Ling Tianyu wasn't wrong. The Fourth Elder and his side needed a righteous pretext to act; he was giving them that chance. They wanted to replace the Book Sea Alliance, and he wanted to resolve Emperor Mountain's current crisis. Mutual aid and support—why not?
Right now the Great Elder had people surrounding Ling Tianyu and his wife, and now the Fourth Elder had turned around to surround the Great Elder and the others. It was perfectly logical; this was called fairness.
In fact, they really ought to thank Emperor Chi Cheng. If not for his attempt to rope Ling Tianyu in, Ling might not have come at all, and would never have known that the internal strife within the Book Sea Alliance was so fierce, even to the point of complete incompatibility like water and fire.
This windfall had been handed to him for nothing; it was truly a fine thing.
