Here is the full list of all 15 Lust for Darkness achievements. It takes around 3-4 hours to unlock all of the achievements on Windows.
Complete all the Side Stories
Complete the Side Story: Yelvertons' Legacy
Complete the Side Story: Arcanes of Lusst'ghaa
Complete the Side Story: Cult of Ecstasy
Go insane
Spend 10 minutes at the cultists' orgy in the Yelvertons' Mansion
Open the chest in Amanda's room
Finish the game
Rescue Amanda from the mansion
Defeat Willard Yelverton
Die
Enter the world of Lusst'ghaa
Get captured by the cultists
Enter the Yelvertons' Mansion
Listen to the full dialogue about the name "Lusst'ghaa"