Here is the full list of all 34 Demonheart achievements.
Max out both Evil and Tactful points.
Max out both Good and Defiant points.
Ask the demonspawn as many questions as you can.
Finish the game as Good Bright and as Evil Bright.
Kiss Ari.
Kiss Sir Brash.
Kill Mark.
Max out both Evil and Defiant points.
Examine Mr. Edgardo after the dinner with Orchid.
Read the book about demonspawn in Chapter 1.
Finish Chapter 5.
Decipher the whole message in Chapter 5.
Finish Chapter 4.
Read all the entries of Rivera's journal.
Max out both Good and Tactful points.
Decipher the message in Chapter 4.
Finish Chapter 3.
Decipher all the messages in Chapter 3.
Finish Chapter 2.
Decipher the message in Chapter 2.
Finish Chapter 1.
Decipher all the messages in Chapter 1.
Molest the demonspawn.