Here is the full list of all 55 GetsuFumaDen: Undying Moon achievements.
Reached the Hellfire Cliff for the first time.
Defeated Ryukotsuki for the first time.
Successfully used Slay on a regular stage.
Reached the Land of Limbo for the first time.
Traversed the Blighted Foothills for the first time.
Completed Commoner difficulty.
Completed Hero difficulty.
Completed Veteran difficulty.
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Used Souls to raise Main Weapon Rank to 48 or higher.
Used Souls to raise Subweapon Rank to 24 or higher.
Used Souls to perform Vitality Up 16 or more times.
Completed up to the last stage without ever using Souls.
Completed the game within 30 minutes.
Completed the game while only picking up main weapons and subweapons at your base.
Defeated Hydra of the Depths for the first time.
Defeated Colossal Centipede for the first time.
Defeated Twin Tempest Demons for the first time.
Defeated Subterranean Spider for the first time.
Defeated Nine-Tailed Fox for the first time.
Defeated Getsu Rando for the first time.
Defeated Evil God Amazu-Tosu for the first time.
Unlocked 5 or more Katana in Refinement.
Unlocked 5 or more Battle Umbrellas in Refinement.
Unlocked 5 or more Dual Wield Weapons in Refinement.
Unlocked 5 or more Bludgeons in Refinement.
Unlocked 5 or more Spears in Refinement.
Unlocked 5 or more Gauntlets in Refinement.
Unlocked Kashiwamaru, the Bloodthirsty Blade.
Unlocked Kagamigusa, the Bloodthirsty Umbrella.
Unlocked Hawkwing, the Bloodthirsty Blades.
Unlocked Foxglove, the Bloodthirsty Club.
Unlocked Uriwa, the Bloodthirsty Spear.
Unlocked Myoga-no-Ken, the Bloodthirsty Gauntlets.
Unlocked all of Getsu Fuma's available Training skills on Commoner difficulty.
Activated Flash 500 or more times in total.
Unlocked 10 or more Mastery skills.
Activated Slay 100 or more times in total.
Died 50 or more times in total.
Activated Demonization 50 or more times in total.
Activated the final level of Demonization 10 or more times in one game.
Reached The Great Wave of Damnation for the first time.
Reached the Misty Peaks for the first time.
Reached the Avenue of Pleasure and Illusion for the first time.
Reached the Frozen Graveyard for the first time.
Reached Cocytus for the first time.