The island of Zara-Nui was once calm and peaceful, protected by the three Toa Zara. The peace was short lived, as evil arose from the Great Lake. Strange mutant begins of unknown origin, the Borzorak and their powerful leader, Lesovak, quickly turned the once pristine lake into a poisonous swamp. They used their Power Spheres to infect Matoran and Rahi on the island. The Toa Zara could barely hold them off, even with the help of the legendary Fire Crafter. They used their spheres to fight the Borzorak and disinfect the Corrupted Matoran. The Borzorak, however, have managed to create the giant Swamp Reaper, a mutant monster, out of wild Rahi. The battle against corruption has begun on the island of Zara-Nui.