The Dark Prophecy

This Achievement unlocks for completing the game. Here's my quick 20-minute walkthrough with commentary.

Intro 0:00
Deep Waters 1:25
Careless Eater 2:45
Open Sesame 3:30
Bookworm 4:05
Potion Master 7:10
The Sword & The Star 9:05
Unattended 11:00
Culinary 12:35
Chamber of Secrets 17:45
Troll Defeater 18:20
Heartless 18:45
Village Savior 19:30

Story related, can't be missed.

After beating the troll, you will receive the Insomnia pollen.

Go into the castle, through to the Wizard's room, and use it on Merlin.

Exhaust all dialogue options and you will beat the game, unlocking this achievement.