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According to Deathbringer, who appears to be something of a leader in the group, the Reapers were originally all various warriors capable of massive destruction. Their skills in destruction however would cause them to be feared by their own people, forcing them into isolation and eventually driving them towards utter madness. After several of these being found each other, they resolved to use their needs for destruction by destroying war throughout the universe through wiping out violent races and destroying things worth fighting over.
The Reapers are composed of a multitude of alien races, virtually no two of which are the same. The only common link among them is that each member is capable of inflicting massive destruction. Very few of the Reapers are ever named, the exceptions being Deathbringer and a dragon-like being named Klaxa. Certain other species were briefly identified by Sixshot, but even he was uncertain over most of them.
In TF Spotlight: Sixshot, the Reapers are shown to have only about six members, but in TF: Devastation, they're shown to actually have countless members, implying that the first six that destroyed the world of Mumu-Obscura where merely a small part of the whole.