Here is the full list of all 32 The Mildew Children achievements.
Finish chapter 3
Finish chapter 2
Meet the Spirit of Apathy
Finish chapter 1
Meet a strange girl
Die while performing rites
Remember Littlegrave's words
Try to walk away from Littlegrave
Finish chapter 4
Finish chapter 5
Finish chapter 6
Finish chapter 7
Finish chapter 8
Finish chapter 9
Finish chapter 10
Finish chapter 11
Finish chapter 12
Meet the Spirit of Contemplation
Meet the Spirit of Loss
Meet the Spirit of Discord
Win a game of Bull's Feast
Kill the hungry wolf
Make your apprentice cry
Escape Death's grip
Overcome Death herself
Save the strange girl
Help the unlucky boy
Make no mistakes during the initiation
Help Lyrida overcome her fears
Make no mistakes on the night of the Dormition
Unlock the good ending
Help your brother realize his plan