Here is the full list of all 12 Song of the Myrne: What Lies Beneath achievements.
Have an item equiped in each of your appearance equipment slots at the same time
Find the secret ending of "The Sorcerer"
Finish the story "The Sorcerer"
Craft an item
Walk Henry with a leash
Unlock all magical skill trees
Beat the Spider's Forest boss. Spoiler: it's a spider
Defeat The Troll
Defeat The Lich
Defeat The Giant Plant
Defeat the final boss
Read a certain grimoire