When Feng Qing Yang detonated his clone in the Title Temple, Duan Ling Tian was stopped by Wu Hong Qing and the others from leaving. He knew he did not have much of a choice left at that time.
Back then, the World Cleansing Divine Water had said, "Little Tian, this independent realm belongs to the Title Temple so there are many Formations. We won't be able to escape even if we join forces. We must leave this independent realm. After that, try to lure them to a mundane realm. Since the space and barriers in mundane realms are weaker, we're 100% certain we can help you escape."