The Miln were signatories to the Treaty of Kammzdast, which governed the use of Orion slaves. Following their rebellion during the Orion War, the Miln fought against the Orion rebels, and were the last to resist them in the final victorious stages. Even after its conclusion in February 13th, 56 BCE (reference stardate -20/5602.13), peace was never formalized.
The Miln withdrew into their cities, remaining reclusive while their society decayed. The Orions couldn't be bothered completely conquering or absorbing the Miln, and no real trade between the two ever took place. The Orions ignored them and burrowed underground to develop and mine.
By the late 23rd century, their crystal cities had fallen into half-ruined states and only legend claimed that the Miln still dwelt within them. (FASA RPG module: The Orions: Book of Common Knowledge)