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