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