The Leech Legion was a contingent of genetically modified Lugubraa, also known as blind berserkers, located within the Unknown Regions.
During a brutal civil war amongst themselves, the Croke brought Lugubraa warriors to their homeworlds situated in the Croke Reach. These Lugubraa showed themselves to be so persistent and lethal that for more than 300 years, they expanded their reach, decimating Croke settlements throughout the region, having escaped the control of those who initially employed them. The Croke then sought assistance from the cloners of Kamino, in the belief that they needed to combat the Lugubraa with their own creations. The Kaminoans engineered a new genetic variation. This new force was dubbed the "Leech Legion" by the Croke, but these clones proved to be just as dangerous to the Croke as the original Lugubraa.
At some point during the Clone Wars, the Leech Legions launched an assault on the planet called Niorde. To defend their planet, several scouts from the native Niordi went to a secret laboratory where they underwent a drastic cybernetic transformation, becoming living weapons. Numerous scouts died during this experiment, and before the survivors were ready for action, the Leech Legions had already taken over Niorde. The remaining cyborg artillery units then attacked the enemy flagship, with only D'harhan, a Niordi living weapon, surviving the battle.