Ennis/ Nol-Ennis

Star Trek: Deep Space Nine
Episode: DS9 413 - Battle Lines

Origin: Gamma Quadrant. Two humanoid factions of the same species who fought for so long they were banished to a prison moon to continue their fighting, able to finally die only if they escape the prison due to special microbes placed in the moon's atmosphere which rebuild injuries and prevent anyone who dies on that moon from leaving it.