"After the ..."Defeat" of our last overlord,and before the Great Cataclysm that destroyed our lands,we fled here"
Gnarl Commenting on how they arrived in the Netherworld
The Minion Exodus is the event that occured 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 dissapeared, leaving them without a guide,forcing them to search for a new master.
Overlord:Raising Hell
Shortly after the Third Overlord got trapped in the Infernal Abyss, the current pregnant Mistress of the Third Overlord left the Dark Tower and left the Minions without a successor, 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 the respective hive members carrying it were distracted during the voyage and the hives (with the exception of the Browns) were lost.
Overlord 2
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 rised again.