So it seems Demacia is still suspect to Demons invading their land.

First Nocturne was considered a terrible demon Garen and Lux's ancestor fought. And now Eve murdering Vayne's parents. Even if petricide wards out magic, spiritual beings can still Terrorize their lands. I suppose this is why Lucians order was created, and I'd like to think that was the same from Lux works with.