Prisoners on a Gamma Quadrant moon, kept fighting forever by artificial microbes that bring the dead back to life.
The Ennis and their enemies the Nol-Ennis were sentenced to a penal moon in the Gamma Quadrant, where they fight a war that can never end. Their captors seeded the moon with artificial microbes that repair and revive anyone killed, so the two sides die and rise again every day.
When a runabout carrying Kai Opaka, Sisko, Kira and Bashir crashed there, Opaka was killed and then resurrected. The microbes now bound her to the moon, and she chose to stay to help the Ennis and Nol-Ennis find peace. Their story is told in Battle Lines.