The Ennis were one of two factions of a humanoid species. The other faction was their mortal enemy, the Nol-Ennis, with whom they were engaged in violent conflict. Both groups were trapped on a moon in the Gamma Quadrant where death was rendered impermanent by microcellular technology that repaired wounds and resurrected the deceased. This remarkable technology only functioned within the confines of the moon's atmosphere. Since no battle was ever decisive, the Ennis and Nol-Ennis were effectively sentenced to an eternal war.
In the 24th century, the Ennis were led by Golin Shel-la, who went along with Benjamin Sisko's attempt to bring the Ennis and Nol-Ennis to peace in 2369. However, Shel-la and his people were ultimately able to think of nothing other than killing the Nol-Ennis, and Sisko's plan failed. Kai Opaka, who had died and been brought back to life by the same technology and was therefore unable to leave the moon, saw her exile as a chance for her to attempt to bring peace to the Ennis and Nol-Ennis once and for all. (DS9: "Battle Lines")