Here is the full list of all 26 Eternights achievements.
Earned all trophies.
Date every dateable character.
Reach max rank with all your confidants.
You went on all of Yohan's dates.
You went on all of Min's dates.
You went on all of Yuna's dates.
You made a promise with Yohan.
You made a promise with Min.
You are a social butterfly.
Going out with 1 person only til the end.
Reached max rank with yourself.
You retrieved the Stone.
You made a promise with Yuna.
You went on all of Chani's dates.
You went on all of Sia's dates.
Reach max rank with a Confidant.
You made a promise with Sia.
You helped Min break through the second wall.
You scavenged 10 city items.
Now you know how to give proper piggyback rides.
You got creative with your hand.
You helped Yuna destroy the first wall.
You explored the city for the first time.
The start of a peculiar bond.
Escaped the shelter.
You awakened your new arm.