Here is the full list of all 21 Seasons after Fall achievements. It takes around 8-10 hours to unlock all of the achievements on Windows.
Explore and make all the flowers bloom.
The End.
Find a place to sleep.
It's not quite the end.
Activate this altar to invoke the Guardian of Spring.
Find the bird, listen carefully and keep very still.
Activate this altar to invoke the Guardian of the Fall.
Activate this altar to invoke the Guardian of Winter.
Activate this altar to invoke the Guardian of Summer.
Bring "something very precious" to the Sanctuary.
Return to an unfamiliar place.
Enter an unfamiliar place.
You're not alone anymore...
Use the Summer fragment to reveal this fresco.
Use the Spring fragment to reveal this fresco.
Use the Autumn fragment to reveal this fresco.
Use the Winter fragment to reveal this fresco.
What's on the surface?