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