The Marauders are a group of superpowered criminals working on behalf of the interests of the villain Mister Sinister. Sinister had long devoted his life to the study of genetics and mutations in particular. Knowing of the existence of an underground community called the Morlocks, Sinister decided that their existence was compromising and diluting the genetic gene pool. To remedy this, he made Gambit, a thief that owed Sinister a favor for restoring balance to his formative mutant powers, to form a group of mercenaries into a team of Marauders that would work for him.
In one of the Marauders' first known activities, based on Sinister's interest in the genetic lineage of the mutants Cyclops and Phoenix, several members of the team hunted Cyclops' wife Madelyne Pryor-Summers and child Nathan, killing Pryor and kidnapping the baby.
Gambit soon led the Marauders into the Morlocks' tunnels, whereupon the mercenaries began a systematic massacre of the population. Unaware of the Marauders' true intent, Gambit tried to stop them, but was only able to save a Morlock girl (the mutant known as Marrow) before escaping. The Marauders were confronted by the Morlocks' allies, the hero team X-Men, along with other heroes such as Thor and Power Pack. Ultimately, the X-Men and the heroes stopped the massacre, but not before significant causalities were sustained by both sides.
Later, another Marauder, Malice, attempted possession of the X-Men ally, Dazzler, but was defeated by Dazzler's friends in the X-Men. The Marauders went on to attack the X-Men's associate Polaris. Sinister had decided, unbeknownst to Malice, that the mutant would make a genetic match with Malice and allow the two to become permanently bonded. It seemed that Polaris had managed to single-handedly and ruthlessly defeat the team, but in fact it was Malice who had taken possession of Polaris and declared herself leader of the Marauders. Malice, in Polaris' body, led the Marauders on several missions, including a failed attempt to capture Madelyne Pryor, who reappeared in California and would be rescued by the X-Men.
When a demon invasion occurred in New York City, Malice/Polaris led a Marauder attack on the hero teams X-Men and X-Factor. Several Marauders who were believed to have died in previous battles had reappeared. The heroes defeated the Marauders, many of whom were killed during the battle, but Malice and Sabretooth escaped. (Both of whom would never regroup with the Marauders since then.) Months later, when investigating Sinister's facilities, the X-Men discovered that Sinister had kept genetic information of the mercenaries in order to create clones of them. Some of the clones were released, but the X-Men defeated them and they died.
With the genetic information readily available, Sinister activated several Marauders once more, this time to chase the mutant Threnody who was escaping her indentured servitude to Sinister. They were confronted by Nate Grey, the X-Man, and in the course of the battle, most of the Marauders were killed. The remaining member, Riptide, was sent back with implanted memories that Threnody was killed as well. Nevertheless, more Marauders later returned, and Threnody agreed to return to Sinister.
Various Marauders have reappeared since then. For example, Sinister disguised several Marauders as agents of the villain Apocalypse in order to try to trick the hero Cable into forming an alliance with him. Another time, Gambit needed to invade another of Sinister's bases, and Scalphunter revealed that all cloned Marauders were genetically programmed to never turn against Sinister on penalty of death. Gambit turned instead to Sabretooth, who was never cloned, and the two invaded Sinister's base after facing several Marauders as guards. Presumably, as long as Mister Sinister remains active, he will always have at hand a cloned army of Marauders in some capacity.
Recently, the Marauders joined forces with the Acolytes, to kill the X-Men and take the Mutant Baby. Several members died, in the process (some members were devoured by Predator X).