From the way it was worded the world (or gods) see Kayku as a second demon lord due to his link with Satan, and were boosting Ars since he's the Hero.
If we use half-assed medical analogies, Ars (white blood cell) manages to kill that link, and turm Kayku from the virus he was seen as (foreign but harmful) into a vaccine (foreign but helpful) in the eyes of the "world".
its been a while since i read the webnovel so do correct me if im wrong in explaining
basically that world has a will of some sort that try its best to kill any outsider entering the wrold but it can't do it directly so they often used guidance (in this case the church) and empowerment (the reason ars is so strong later on compared to any heroes before him) because kakyu was the strongest demon it basically give all kind of empowerment it can to ars when their duel is close and kakyu notice it so he planned to died by ars hand because if this keep going on he doesn't know what the world will throw at him next (with extra since he is an outsider he can't have children as well) ars later on notice this and decided to change direction (since he still love kakyu as his father even thought at this point kakyu already tell him he is not his father) to basically fool the world by going in to interdimension with kakyu and remade his existence there that it make the world think kakyu was a resident of the world and it work because the world already give so much enchantment to him so kakyu manage to survive and later on have a daughter (because he is a resident of the world now) which ironically fell in love with ars son
well i was feeling sleepy noww so if i misstype sorry
well hope that explain everything and good night
