The nucleogenic creatures
The nucleogenic creatures were intelligent beings from another dimension.
In the 2370s, a Starfleet ship, the USS Equinox, was shown how to summon the beings by the Ankari and were given the technology. When they summoned another creature and attempted to study it, it died. It was found that the creatures nucleogenic energy could greatly enhance the power of a warp engine. The Equinox crew killed more and more creatures, travelling over 10,000 light-years in less than two weeks.
The aliens soon took revenge on the crew and began attackes on the Equinox. The ship were saved by the USS Voyager. After Rudolph Ransom discovered that the crew of Voyager would not agree with what they had done to the creatures, he decided to take Voyager's field generator. Kathryn Janeway sought the Ankari and used them to translate the aliens language. She promised them that she would deliver the Equinox to them. (VOY: "Equinox, Part I", "Equinox, Part II")