The Crusade of the Shining Justice is a conflict occurred prior the events shown in Overlord: Fellowship of Evil and after the fall of the Glorious Empire and the establishment of the Fourth Overlord's evil empire at the end of Overlord II. The Crusade was a massive military campaign started by the newborn organization of the Shining Justice against all the evil creatures in the World, especially the bellicose Minions and their evil master, the Fourth Overlord. Not much is known about the details of the crusade, but it's well known that at the end of the conflict the heroes of the Shining Justice succeeded in killing the evil Overlord and reducing his mighty minion horde to a dim shadow of its former self.
However, the warrior spirit of the minions wasn't broken by this defeat and they wandered through many lands in search of a powerful magic artifact that could help them get revenge for the death of their master. Finally, with the help of the Unknown Lord, Gnarl found the artifact, the Book of Evil, and with it gave birth to the nefarious Netherghuls.