The beginning of the demon wave was a historical event, as well as a critically important event in the Holy War's timeline.
The demon wave's arrival signified that the Chaos Abyss had successfully connected to the mortal plane. It would be very difficult for any Chaos Abyss resident to come to the mortal plane when it wasn't a time of Holy War, and the Chaos Abyss resident would also be quite weakened and easily kicked out of the mortal plane. But now, not only was establishing portals easier, even powerful demons would no longer be restricted by dimensional laws in the mortal plane now.
However, there would always be demerits together with benefits. Previously, due to dimensional laws' restrictions which forcefully suppressed those from the Chaos Abyss, the Demon Lords were only able to send out incarnations, clones, descendants, and so on. Even if they suffered some losses in the mortal plane, it wouldn't mean their actual death.
