The chaos of outer demons.
This is almost an inevitable occurrence.
Duncan wants to break through the upper limit of this world, which will certainly lead to spatial tremors. Then, this world will resemble a bubble about to burst, so long as a single crack appears, those Outer Heaven Demons, who have been watching closely, will swarm in.
Even when no one raises this world's limit, the Outer Heaven Demons will exploit the opportunity to sneak in, but such situations usually involve only one at a time, unable to cause significant destruction.
Yet this time Duncan used the Three Corpses as bait, causing a thousand Outer Heaven Demons to flood in all at once.
Although Duncan killed a significant portion of the Outer Heaven Demons, some still managed to escape amidst the chaos. They even abandoned the possessed bodies, fleeing as invisible demon shadows.
"Something's wrong!"
