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