Their original plan was to lure the large number of Demon Fiends in the city over and kill as many as possible. They held no illusions about being able to kill the Corpse Transformation Realm Demon Fiends among them.
It was incredibly difficult for someone in the Corpse Transformation Realm to kill another of the same realm, unless they were willing to go all out.
But under these circumstances, even if they escaped, they would just run into other Demon Fiends. No one would use all their trump cards; everyone would hold something back.
After all, what could be more important than their own lives?
So, all things considered, the proposal was an excellent one.
If they could lure all those Great Demon Fiends into creating chaos among themselves, they could strike at the opportune moment and reap the rewards without any of the risk.
