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