The Holy War is a war that took place to defeat the World Enemy - The Gluttony.
Chronology
To fight against the World Enemy - The Gluttony (Rou), the Lumen Holy Kingdom allied themselves with the Kirika Empire, the Sternbild Kingdom, the Atarakua Demon Empire and the Estgrand Beast Kingdom.
This war takes place in dungeons owned by Rou, principally the Funeral Volcano. The war lasted about two days.
Parabellum emerged victorious in this battle, killing most of the Heroes and converting some of them.