Noncorporeal criminals from the Ux-Mal system who took over Troi, Data and O'Brien in an attempt to escape their prison.
The Ux-Mal are noncorporeal beings from the Ux-Mal star system who can exist without bodies and take control of the bodies of others. Their society punished certain criminals by sending their disembodied consciousnesses to a penal colony on a moon of Mab-Bu VI.
In Power Play, a group of these convicts lured the Enterprise by posing as the spirits of the crew of the long-lost USS Essex. They took over Deanna Troi, Data and Miles O'Brien, seized Ten Forward and held its occupants hostage while trying to free the rest of the prisoners. Picard eventually forced them to return to the moon.