Beings from a realm beyond normal space whose bodies the crew of the USS Equinox killed and used as fuel to speed their journey home.
Nucleogenic lifeforms live in a space separate from ours. Their remains hold a substance that can boost a starship's warp drive. When provoked, they can tear through spatial rifts and attack humanoids directly, leaving them badly wounded.
In Equinox, Voyager found the battered USS Equinox under Captain Rudolph Ransom. Janeway discovered that Ransom's crew had been capturing and killing the beings to power their engines. The aliens attacked both ships in revenge, and the conflict ended with the loss of the Equinox.