The Hidden Moon Sect Leader was motivated to join forces with other realms to divide the Southern Domain after witnessing Xiyue Immortal Emperor make a move.
Relying solely on their Barbaric forces, they could seize the entire Southern Domain, but it would require a huge sacrifice, and they're most concerned that the Southern Domain might act desperately and destroy the seals in the Sunset Mountain Range again.
This time, the Southern Domain has been cautious, only breaking seals in a few Forbidden Lands. If, by chance, they break all the seals in the Forbidden Lands next time, it would be beyond the Barbaric realm's ability to handle.
Although the likelihood of this is very small, it must be guarded against.
Of course, sharing the Southern Domain with other realms would reduce the resources and Qi Fortune they gain, but with the help of other realms, it would be much easier to completely annihilate the Southern Domain.
