The Marauders were a group of ragtag pirates and invaders who attacked all the Nine Realms. They are composed of various races, including Kronans, Sakaarans, Easik, Yrds and R'malk'i, many of them considered lowlifes by the more cultured and civilized inhabitants of the realms.
The Marauders formed in order to seize the opportunity provided by the destruction of the Rainbow Bridge. Without the bridge, the Asgardians were confined to their home realm, leaving the other Nine Realms completely undefended. The Marauders then set up an invasion of different realms and planets, murdering and looting whatever they could find.
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The Asgardians cornered the last Marauders in Vanaheim. Thor personally intervened and easily killed one of their champions, the Kronan Marauder, prompting the surviving Marauders to surrender. These criminals were taken to Asgard to be imprisoned in its dungeons.
During a prison break engineered by the Dark Elves in order to provide a distraction to take the Aether from Jane Foster's body, the Marauders were able to procure weapons and begin battling the guards. The remaining Marauders died in the fight, though some survived and remained imprisoned.