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