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