Here is the full list of all 39 Eldritchvania achievements.
Find the Necronomicon
Learn the Eldritchblast Spell
Enter into the Obelisk Garden
Kill the Nullug Watcher
Kill the Wrath of Yig
Defeat Father Dagon and Mother Hydra
Defeat Cultist Leader Maurice
Survive the Horror of The Gate
Kill a spectre in the obelisk garden.
Kill a Genk before it can run away
Kill Hydra and Dagon without the Sphere of Dagon
Die a painful death
Find all health upgrades
Find all Sanity upgrades
Find all semblance upgrades
Complete the journal
Learn the Worm Spell
Find the Nullug Talisman
Find the Kadathian Sphere
Assemble the Cultist Key
Collect Sphere of Dagon
Find the Armour of Protection
Find the Sacrificial Dagger
Find the Whip
Recieve help to break the cycle
Get healed by Priest Marcus 20 times
Use a holy beacon
Reach the gate
Break the seal of The Gate
Add 20 entries into the journal
Help Bruce return to the Blackhorn Church
Complete the Game
Escape the island in less than two hours
Enter into the Voynich Glade
Enter into the Sunken City
Solve the puzzle of the six snake heads
Beat the game with 100% completion