Here is the full list of all 18 Elder Chaos achievements.
Finish A Map with 3 stars
Use Deity Skill 5 times in a game
Occupy an area with 5 servants
Win a game with 6 Fragments found
Finish a game with no investigator on board
Threat reaches to 1000
Complete all chapters
Win a game with Stella Skill
Eliminate 2 more investigators and servants at one time with Melt
Make 10 more investigators insane in a game
Have 10 or more Misanthrope on board
Collect all stars
Win a game with no facility destroyed
Summon 2 Hounds from Twin Tower
Have 15 investigators on board
Reveal Blood Rain Omen
Complete Chapter 1
Complete Chapter 2