Here is the full list of all 15 Eldritch achievements.
Acquire the Soul of Dagon.
Acquire the Soul of Nyarlathotep.
Acquire the Soul of Cthulhu.
Escape the Mountains of Madness.
Finish the game in New Game+ mode.
Deposit 9,999 artifacts in the bank chest.
Perform the Ritual of Binding.
Perform the complete Ritual of Binding.
Read the complete History of the Library.
Survive for 3 minutes after Azathoth appears.
Finish the game in under 10 minutes.
Finish the game without taking any damage.
Make friends with a star-spawn.
Destroy a lizardman statue.
Free 500 trapped souls from the Asylum.