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