Here is the full list of all 54 Ys IX: Monstrum Nox achievements. It takes around 50-60 hours to unlock all of the achievements on Windows.
Clear the Final Chapter.
Defeat the ultimate Lemures, Vakh Medios (excludes Time Attack).
Use Flash Move 100 times.
Attain 100% of landmarks discovered.
Have obtained a total of 2,000 NOX points.
Achieve an S on all Grimwald Nox.
Attain 100% of Ritual Relics enhanced.
Clear all Grimwald Nox.
Attain 100% of all "Materials" entries in your Journal.
Explore Balduq with the Hawk as leader.
Explore Balduq with the Doll as leader.
Explore Balduq with the White Cat as leader.
Clear Chapter 8.
Travel a total distance of 300 krimelye.
Explore Balduq with the Renegade as leader.
Clear Chapter 7.
Attain 100% of graffiti discovered.
Use Flash Guard 100 times.
Clear Chapter 6.
Inflict Break 1,000 times.
Explore Balduq with the Raging Bull as leader.
Use Extra Skills 100 times.
Clear Chapter 5.
Use Boost Mode 100 times.
Defeat 2,000 monsters.
Defeat 1,000 Lemures.
Clear Chapter 4.
Explore Balduq with the Crimson King as leader.
Clear Chapter 3.
Clear Chapter 2.
Achieve an S in a Grimwald Nox.
Clear Chapter 1.
Clear the prologue.
Begin your escape from the prison.
Finish "Balduq Prison" by Adol Christin in its entirety by earning all achievements.
Complete the map of Balduq.
Attain 100% of all treasure chests.
Attain 100% of all quests.
Attain 100% of all "People" entries in your Journal.
Attain 100% of all "Monsters" entries in your Journal.
Attain 100% of Affinity with all characters.
Attain 100% of Azure Petals found.
Attain 100% of shops discovered.
Have 1,000,000 Gold in possession.
Unlock all 13 recipes at the Dandelion.
Reach max level with all skills.
Forge an Ultimate Armor.
Forge Ultimate Weapons for all Monstrums (6 in total).
Reach level 99 with any character.
In Time Attack, defeat the boss in 30 seconds or less.
In Time Attack, defeat the boss without taking damage.
Complete Boss Rush.
Complete Boss Rush without retrying.
Clear the game on Nightmare difficulty or above.