The Minion Exodus is the event that occurred after the events of Overlord: Raising Hell but before the Great Cataclysm, That consisted of the Minions fleeing to the Netherworld when the Third Overlord was trapped in the Infernal Abyss and his son disappeared, leaving them without a guide, forcing them to search for a new master.
Red: Wandered off because there wisps distracted them leading into a volcano and they made their home in the Norburg Sanctuary.
Greens: Smelled something reeking so they disappeared with their hive to the Everlight Jungle.
Blues: Felt so lost without an Overlord so they went off with their hive to the empire sewers where most of the water gets accumulated making it their most favored area.
Shortly after the Third Overlord got trapped in the Infernal Abyss, the current pregnant Mistress of the Third Overlord disappeared from the Dark Tower along with the successor and left the Minions without a guide, forcing them to leave the Dark Tower and reach the Netherworld, leaving some minions to guard the Tower. The Hives were brought away from the tower under Gnarl's order(but not, strangely, the Tower Heart, had it been taken, it could have prevented the great cataclysm but because the energy was mostly consecrated on sitting in the spawning pits for a long time meant it would be harder to drag the power away from the pits. So they had to leave the Tower Heart behind), but the respective hive members carrying it were distracted during the voyage and the hives (with the exception of the Browns) were lost.
just before the events of Overlord 2, the Exodus was completed and the minions focused fully on searching a new master and possibly find the Third Overlord's Son, checking the Dark Tower once in a while, in case they found a new master, they would come back. until the Great Cataclysm caused by Florian Greenheart destroyed the Tower. Shortly after that, the Browns found the Fourth Overlord in Nordberg and the Exodus was over, as the reign of the overlord rose again.