The Nol-Ennis were a faction of a humanoid species, and were the mortal enemies of the Ennis faction. Along with the Ennis, they were trapped on a moon in the Gamma Quadrant where death was forgone by microcellular technology that only functioned within the confines of the moon's atmosphere, effectively sentencing them to an eternal war with the Ennis.
In the 24th century the Nol-Ennis were led by Zlangco, who did not trust the Ennis leader Golin Shel-la. Benjamin Sisko led a failed attempt in 2369 to bring the Ennis and Nol-Ennis together; their distrust of one another prevented either group from thinking about anything but killing the other side. Kai Opaka, who had died and been brought back to life by the same technology, stayed behind in an attempt to bring peace to the Ennis and Nol-Ennis once and for all. (DS9: "Battle Lines")