Resonators were weapons designed by the Imperial engineer Umak Leth to subjugate the Marasan people.
Since all Marasans had sophisticated computers linked directly to their brains as cyborg implants, Leth discovered that high-energy resonance waves which disrupted computers would directly affect the Marasans themselves. By developing resonators which set up feedback in a Marasan's mind, Leth created weapons which could cause disorientation and pain in a Marasan.
Initial models were hand-held devices which could be used against single Marasans, but larger, more powerful resonators could torture and incapacitate a planet full of Marasans. The threat of such collective punishment allowed the Empire to conquer the Marasan homeworld of Marasai, as well as the Marasan colonies in the Marasa Nebula.