Here is the full list of all 34 Eris and the fading kingdom achievements.
Defeat Fae
Defeat Spedig
Defeat Trio Of The Tower
Defeat Drake
Defeat Wear Wyrm
Defeated the Hero's Phantom
Defeat Weordan
Fail to consume Weordan
Sacrifice yourself to save the kingdom
Defeat the Green Man
Find the key to the library
Consume an enemy for the first time
Consume every type of enemy
Defeat Dhu Guisch
Defeat Marool
Escape the Will-O-The-Wisp
Defeat the Beast
Defeat Ren
Defeat Big Ears
Defeat Jack Of The Lantern
Defeat Bregdi
Defeat Knucker
Defeat Stoor Worm
Defeat Kelpie
Defeat Nessy
Defeat Cathach
Defeat the Salmon Of Knowledge
Defeat Ellen Trechend
Defeat Puca
Defeat Onchu
Defeat Muridris
Defeat Fae Lad
Defeat Dullahan
Defeat the Bomere Fish