Even when she hadn't met Alister and hadn't learned God Slaying Magic, Zanna was powerful enough to look down on every living being in Fantasia.
The so-called Demon King was nothing but an unkillable cockroach to her. Although she couldn't kill them, it was easy to defeat and seal them.
However, throughout history, it was rare for Zanna to intervene in the battle between heroes and Demon King. She would only do so if the heroes were all dead and the Demon King was liberated from the territory curse.
Otherwise, Zanna would rarely make an appearance in the outside world since she mostly protected the spirit race in secrecy. Aside from the liberation of the Demon King's territory curse, Zanna would also only intervene if the spirit race was facing a great crisis.
It just so happened that in this millennium, not only did her spirit race face a great crisis, but the heroes were also all eliminated. Even eight of the nine heroes had become fallen heroes.