Here is the full list of all 26 The Legend of Heroes: Trails in the Sky the 3rd achievements.
Complete Chapter 8.
Finish the game on Easy or above.
Finish the game on Normal or above.
Complete all Star Doors.
Defeat all Abyss fiends.
Complete Chapter 7.
Complete Chapter 6.
Complete all Sun Doors.
Complete all minigames on any difficulty.
Carefully examine nearly every chest that you come across before the final chapter.
Complete all Moon Doors.
Complete Chapter 5.
Complete Chapter 4.
Complete Chapter 3.
Complete Chapter 2.
Complete Chapter 1.
Complete the Prologue.
Complete the Farewall Arena on Nightmare.
Complete "Who Wants to Be a Mirannaire?" Maniac mode.
Read all of the books in the Garden.
Read a book you wish you hadn't...
Obtain a special quartz from brave deeds.
Obtain a special quartz from cowardly deeds.
Finish the game on Hard or above.
Finish the game on Nightmare.
Beat the ultimate opponent on the highest difficulty.