Old Miltia is the original Miltia that was lost in the Abyss in the Miltian Conflict. Old Miltia holds the Original Zohar and Labyrinthos. The U.M.N. gate for Old Miltia is sealed in the Abyss until the Y-Data, which was entrusted to the 100 Series Observational Realian MOMO, is used by Albedo. After the link between Cecily and Cathe and the Zohar is destroyed, the Ω System awakens and destroys Old Miltia from the inside out. Next, the Ω System is completely destroyed with the annihilation of Proto Ω, which leaves the Original Zohar as the only remaining piece of Old Miltia. That, in turn, is sucked up by Abel's Ark.
Miltia was one of the autonomous worlds occupied by the people of the Immigrant Fleet. This was also the homeworld of the Uzuki family before the Miltian Conflict forced them to relocate to Second Miltia.