After the Cerberus Attack on the Citadel, Liara T'Soni ask you if you check Ardat-Yakshi Base at Planet Lesuss (Nimbus Cluster/Mesana)
You get this for completing the Sidequest (=Achievement is missable) Kallini: Ardat-Yakshi Monastery. You also get the the Sixth Asari Fleet (+90 War Assets), when you complete this Quest