Electromagnetic lifeforms that live inside Class J nebulae, one of which was accidentally carried aboard Voyager and seized control of the ship, as Neelix recounts to the Borg children in 2376.
In the episode The Haunting of Deck Twelve, Voyager shuts down main power and Neelix keeps the four Borg children calm with what he presents as a ghost story. Months earlier, he says, the ship had collected deuterium from a Class J nebula, and the bussard collectors destabilised it.
A being made of electromagnetic energy was drawn aboard and found itself homeless when the nebula dispersed. Moving through the circuitry, it caused malfunctions, injured crew members and eventually took over environmental controls, using fragments of the computer's voice to speak.
Janeway, the last person conscious on the bridge, persuaded it that killing the crew would leave it stranded, and promised to find it a new home. It was then confined to a section of deck twelve. The power shutdown in the framing story is the moment Voyager releases it into another Class J nebula.