These achievements are earned by progressing through the main story:

• Start of the Journey
  Talk to Sul to begin the adventure.
  
  
  
  
• Failed Hero
  Defeat Myrtly
  Reward: First Resurrection Flower.
  
  
  
• Specter of Sin
  Defeat Dhog'Se.
  Reward: Afterward, go down, and right again to find hidden cave and talk to Kiddo to get new weapon Sword.
  
  
• Path of the Flame
  Defeat Daredemian.
  Reward: New weapon Bow.
  
  
  
• Metal-Eating Monster
  Escape from Bulgasari.
  Reward: New weapon Club.
  
  
  
• Saboteur
  Defeat Red Fugitive Soul.
  Reward: New weapon Umbrella.
  
  
  
• Plans
  Defeat Gusionse.
  Reward: New weapon Lamp.
  
  
  
• Puppet
  Defeat Corrupted Sul for the first time.
  Reward: New weapon Fan.
  
  
  
• Cries of Pain
  Escape from Oxy's Thurible of Darkness.
  
  
  
  
• Comrade
  Help the guardians to recover all lost abilities.
  
  
  
  
• Purification
  Defeat Corrupted Sul a second time.
  
  
  
  
• Punishment
  Defeat Corrupted Oxy.
  
  
  
  
• Faith
  Defeat Faith.

These achievements are earned simply by obtaining all perks

• Skilled Pathfinder
  Obtain all Pathfinder Perks.
  
  
  
  
• Skilled Messenger
  Obtain all Messenger Perks.
  
  
  
  
• Skilled Reaper
  Obtain all Reaper Perks.
  
  
  
  
• Legendary Guardian of Death
  Obtain all Perks.

These achievements are earned by completing side quests and explorations

• Hidden Technique
  Complete a side quest from an NPC and learn the hidden perk from them (more information in Technical Death Guardian achievement).
  
  
  
• Technical Death Guardian
  Complete 3 side quests and learn the 3 hidden perks.
  
  
  
  Quest 1: The Hidden Mouse in the Death Tavern
  In the Death Tavern, go to the area where you would normally press up to enter the portal room. Instead of pressing up, press the toad charge button to activate a secret room. Inside, an NPC will ask you to kill rats in the sewers. Defeat the rats, and return to complete the quest.
  
  
  Quest 2: Fifty Monsters in the Frozen Plateau Dispatch Station
  In the Frozen Plateau Dispatch Station, you’ll find an NPC outside. Talk to them, and they’ll ask you to kill 50 monsters in the area. Defeat the monsters and deliver the reward back to the NPC to complete the quest.
  
  
  
  Quest 3: The Styx Doll in the Underground Waterway
  In the Underground Waterway you’ll find a girl named Dusky, she will ask for a doll. To find the doll, you need to take a secret path after defeating the Ghost Bride which can be accessed by climbing the rightmost wall of the boss room. Take the doll and return it to Dusky. The room with this doll also room for the secret boss fight Corrupted Deller (the boss will only spawn after you can access The Land of Abundance).
  
  After completing these three quests, you’ll be taught three hidden perks, learn all of them and the Technical Death Guardian achievement will unlock!
  
  
  
• Greedy Person
  Defeat the Corrupted Deller (The boss room is located where you get the doll for Dusky and only spawn after you can access The Land of Abundance).
  
  
  
• Gap
  Defeat a hidden Elite Guardian (Fan needed to enter).
  
  
  
  
• Kkokdu
  Defeat all hidden Elite Guardians.
  
  
  
  
• Memory Fragment
  Find the first mirror fragment in the Reincarnation Station.
  
  
  
  
• True Memory
  Collect all 6 Mirror Pieces (Locations include Reincarnation Station, Crimson Mines, Frozen Plateau, Misty Forest, Underground Waterway, and Stone Mountain Observatory).
  
  
• Reincarnation
  Collect all Reincarnation Flowers by defeating mini-bosses in each area.
  
  
  
  
• Loyal Guardian of Death
  Interact with all Dawn Tombstones.
  
  
  
  
• Guardian of Death of Guidance
  Guide all Fugitive Souls.
  
  
  
  
• Belief
  After defeating corrupted Oxy, check the banner in the room’s left corner.
  
  
  
  
• Friend
  In The Thurible of Darkness, before fighting Faith, go left and climb the platforms and go right to find this hidden path, follow the path and then interact with the hat in the secret room.

These achievements are earned simply by completing all regions 100%. If you need guidance to 100% every regions, you can check this complete map [demajen.co.uk] by youtube.com/demajen

• Path of Souls
  Kingdom of Yama 100% complete.
  
  
  
  
• Waterway of Delusions
  Underground Waterway 100% complete.
  
  
  
  
• Absent Mountain Range
  Stone Mountain 100% complete.
  
  
  
  
• Slothful Forest
  Misty Forest 100% complete.
  
  
  
  
• Mines of Greed
  Crimson Mines 100% complete.
  
  
  
  
• Land of Wrath
  Barren Grounds 100% complete.
  
  
  
  
• Plateau of Hatred
  Frozen Plateau 100% complete.
  
  
  
  
• Land of Impurity
  Land of Abundance 100% complete.
  
  
  
  
• Purgatory
  All regions 100% complete.

To unlock these achievements, you need to finish the game multiple times and follow specific rules:

• Standstill
  Achieve Bad Ending: Finish the game without completing the Reincarnation Flowers or Mirror Pieces.
  
  
  
• Another Start
  Achieve Good Ending: Finish the game after completing only the Reincarnation Flowers.
  
  
  
• Lost Princess
  Achieve True Ending: Finish the game after completing all Reincarnation Flowers and Mirror Pieces.

Finally, here are the achievements that require skill and patience:

• Absent Dawn
  Clear the game without learning any other Perk except Duoxini (1st Perk for transformation).
  
  
  
• Immortal
  Clear the game without dying.
  
  
  
  
• Rush
  Complete Normal Mode in 3 hours.

You can unlock all three endings on the same save file by following this process: First, complete the requirements for the first ending and finish it. Then, reload your save and fulfill the requirements for the second ending (without completing the third ending's requirements). After that, reload your save again and complete the requirements for the final ending.

That’s it for the guide! I hope this helps you unlock all the achievements and enjoy the game to its fullest. If you have any questions or spot any mistakes, feel free to leave a comment.

Good luck, and have fun playing this hidden gem of a game!