This key item grants access to the ENM battle in The Shrouded Maw (Pso'Xja). It's obtained by trading a Florid Stone (rare/ex) to the Mithra at the Tenshodo HQ in Lower Jeuno. Florid Stones can be obtained by killing Diremites in Pso'Xja. It seems you can obtain the Astral Covenant as soon as you reach CoP mission 3-5 "Darkness Named"