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