Here is the full list of all 12 Song of the Myrne: What Lies Beneath achievements.
Walk Henry with a leash
Unlock all magical skill trees
Craft an item
Have an item equiped in each of your appearance equipment slots at the same time
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
Finish the story "The Sorcerer"
Find the secret ending of "The Sorcerer"