This can be unlocked after you have
Enter Madain Sari with Dagger in your party. Go to the porch in Eiko's house and Lani will run away. Exit Madain Sari and return with a party that doesn't have Dagger, or Amarant. Speak with Lani twice and enter the nearby small room, walk towards the screen and you should have a ! pop-up which will prompt some messages on what to do.
Head to Eidolon Wall in Madain Sari and you'll see the camera angle has changed from before. Walk right around the wall once and when you return to the entrance, you will hear a chime (need to have sounds effects on). Turn around and walk around the wall left until you hear the chime again. Turn around and walk right until you hear a chime and so on. Repeat this until you get a message that HP and MP is restored and status effects are removed (which should be after 9 times). Afterwards, head right and read every inscription on the mural. By returning to Ifrit's inscription and reading it again, you will learn Dagger's true name and unlock the achievement
edit : I went into config and turned sound off but left sound effects on, much easier to listen for the chimes from JamoneTheAfro in comments