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