Here is the full list of all 64 Our Darker Purpose achievements.
Complete Chapter 3
Finish off Specialist Fenwick with at least four patients unconscious
Defeat Cyndar's Timekeeper without using a time bubble
Finish Sudsy the Inanimate with his own glacial spike
Donate 500 credits to the Guidance Counselor
Donate 100 credits to the Guidance Counselor
Complete Chapter 2
Reach the end of Chapter 2 without using a vending machine
Defeat Regan while holding max juice boxes
Defeat Goneril twice
Defeat the Candelabra of Longing without leaving the light
Defeat 500 enemies in the Edgewood Home for Lost Children
Beat the game five times
Complete Chapter 1
Collect 35 items over multiple playthroughs
Collect and hold 40 tokens in a single playthrough
Meet your demise in Edgewood again and again and again
Meet your demise in Edgewood
Collect both the Gourmet Turtle and the Wandering Crab in a single game
Deal over 400 damage with a basic attack on an enemy
Defeat Frothy the Dragon without taking damage
Finish off Some Harmless Markings with a chalk cloud
Defeat The Box of Nice Things with four or more presents still alive
Please sir - defeat the Cauldron of Yesterdayers before A Second Helping
Heal only a single damage with a juice box
Donate 1500 credits to the Guidance Counselor
Defeat an Archivist
Collect the Broken Mirror and the Trapstem Roses in a single game
Defeat The Obsolete Operator with five or more trash can Broggs still alive
Defeat The Frozen Few without failing a maze once
Defeat Goneril before destroying any buildings
Defeat The Disciplined King in under 75 seconds
Defeat The Friendly Foot Stools without taking damage
Defeat Regan three times
Roll while holding the Fragile Roly Poly and the Candelabra Matchsticks
Carry ten or more consumables at once
Donate 5000 credits to the Guidance Counselor
Complete a sensitive floor while under the influence of Severity
Defeat a feral cat champion
Defeat The Virtuoso while confused
Defeat The Winged Overseer with 10 or more small birds still alive
Defeat A Leafy Oversight after letting him absorb 6 or more spores
Defeat The Brick Pile without letting any slime return
Defeat Dr. Bloodfather with less than 13 health remaining
Defeat The Administrators four times
Drink 500 Juice Boxes
Donate 10000 credits to the Guidance Counselor
Deal over 1000 damage with a basic attack on an enemy
Complete the game on Hard Mode
Complete Chapter 4
Uncover both Goneril's Mallet and Regan's Lunchbox
Reach the end of Chapter 3 without rolling
Save 80 tokens in a single playthrough
Collect every single piece of lore
Defeat A Buoyant Double after killing three Frosty Doubles
Defeat Better Cordy without hitting it with basic attacks
Defeat The Imaginaries with less than 50 move speed remaining
Defeat The Administrators after spawning 13 extra enemies
Defeat His Natural Defenses after killing 21 spawned enemies
Reach the end of Chapter 4 in under an hour
Complete Hard Mode without using any juice boxes
Complete Hard Mode in under an hour
Get a ten game win streak
Complete all other achievements