Light

Light was an energy being that took over the crew of the USS Ventus and forced them all to march endlessly and to find new people to march.

Over 1000 years earlier, Light happened upon a military academy on the planet Ontekelon and fell in love with the marching and the pretty uniforms. It took over everyone at the academy and forced them to march. Eventually the Ontekelon military decided that sealing it up and obliterating everyone inside was all they could do. Light was buried in the rubble. Sometime after that, the entire civilization wiped itself out in a series of non-Light-related wars.

The Federation colonized Ontekelon in the 22nd Century. Then in the 23rd Century, the USS Ventus delivered an archeological team to a dig at the site of the military academy. The dig freed Light, who sensed the military mindset in one of the Ventus' officers. Light entered the officer, who then went back to the Ventus, which then broke orbit and headed off into space. Soon thereafter, Light was in control of the entire ship and once again enjoying marching.

Unfortunately for Light, the Ventus happened upon the SS Clydesdale. None of the crew of the Clydesdale wanted to march except for Ronnie Harper, who had been in Starfleet. Light took Ronnie has its host and continued controlling the Ventus crew until Mike Harper grabbed Ronnie and beamed her back to the Clydesdale. Dr. Janet Corbair was eventually able to lure Light of out Ronnie and into a storage container with the promise that it could watch videos out people marching. Corbair then took Light to Antares and traded it for a working tricorder. ("Busted" [TOS])