Class J nebula lifeform
The Electromagnetic life-form was an intelligent, gaseous entity that was residing in a Class J nebula in the Delta Quadrant. The USS Voyager used it's Bussard collectors in the nebula and accidentally abducted the lifeform. The Electromagnetic life-form responded with hostility by attempting to alter the ship's atmosphere to better suit it's home environment while similtaniously changing the ship's course in an attempt to get home.
The electromagnetic life-form's alterations caused power outages and forced the evacuation of many areas of the ship, including the bridge and sickbay. Several crew members reported injuries as a result of system malfuntions cause by the entity, included amoung the injured were; Gibson, McMann and Mulcahey.
From a temporary command post in main engineering, Captain Janeway was eventually able to arrive at a uneasy discourse with the creature, with the it using the voice of the ship's computer to communicate. The lifeform requested to be returned to it's home nebula and Janeway agreed. However, when Voyager returned to the nebula it was found to have dissipated into space. The Electromagnetic life-form was outraged and demanded that the ship be evacuated, Janeway informed it that it could not run the ship without a crew and to run a diagnostic to confirm that fact, the lifeform insisted they evacuate and again Janeway agreed to its demands. While leaving the ship with Chakotay the entity shut the doors between them, isolating Janeway on the ship. It had run the diagnostic and wanted Janeway to stay and maintain the ship's systems, she refused at risk of her own life. Without any other options remaining the Electromagnetic life-form agreed to let the crew of Voyager find it a new home.
During it's stay it resided on deck twelve, causing the area to be off limits, this started rumors of ghost and monsters amoung the ship's children. Voyager later found a new home for the creature and while it was leaving the ship it again caused power outages. As a result much of the ship was in darkness, during this time Neelix was keeping the Borg children entertained with a ghost story of the events. (VOY: "The Haunting of Deck Twelve")