Here is the full list of all 30 Once on a windswept night achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
Let yourself forget everything.
Lycoris gets a little too possessive.
Profess your love for Madeleine.
Profess your love for Daffodil.
Take the traveller's name for your own.
Make yourself the third Madeleine.
Multiply the number of daffodils in the story.
Arrange the actors back on the stage.
Wait for a play that's already over.
Reach your happy ending.
Return to the library when Lycoris needs you the most.
You remember who you are.
Read a lonely spirit's story.
Name yourself after a certain flower.
Try to steal Lycoris' most precious possession.
Bring the story to a premature close.
Learn Lycoris' favourite flavour of tea.
Read Daffodil's story.
The traveller refuses to tell Daffodil the truth.
Offer Daffodil a few too many compliments.
Answer Lycrois' question incorrectly.
Return to the library for a second time.
Read Madeleine's story.
Madeleine refuses to share her story.
D-Did you just insult a girl's cooking?! You monster!
Throw caution to the wind!
Listen to a mysterious voice.
The traveller leaves the church early.
Start a new story.
Lycoris welcomes you to her library.