The Messenger

In the second half of the game or the end game, you will need to collect six different tunes/keys to unlock the melody. I'll try to write a small compilation here. When you press back , you can check the Keys you are missing.

Key of Hope
Probably the first one for many of us. Take the portal to Autumn Hills (left-most portal) and go left until in your map you can see an unexplored area beneath you. Complete this for the Key.

Key of Strength
Quite simple unless I'm missing something. Go to the top of Searing Crags (where Colos and Suses are/were) in the future to pick up a Power Thistle. Return to same place after going back to the past. Talk to C & S and the will, ehm, kindly show you the way to a new area in Searing Crags. Complete it for the Key.

Key of Love
Separate guide for this one:


Key of Chaos
New area of Underworld, which can be accessed a bit right from the Searing Crags portal. You will need the Lightfoot Tabi from Sunken Shrine (see Key of Love) to get to this area.

Key of Symbiosis
This has a whole lot of prerequisites and steps. The Key is the reward for completing Elemental Skylands and for that you need to do the following:
1. Go through Dark Cave. There is an achievement tied to completing it without the Power of True Sight so bear that in mind. Dark Cave can be found by going down in the middle of the Catacombs.
2. Go through Rivière Turqoise (unlocked by going through Dark Cave) and beat the boss there to receive Magic Firefly.
3. Go to the boss room of Quillsroom Marsh in the past to unlock the monk.
4. Go to the middle of Glacial peak in the past to unlock the two lamps, then go to the right to swap back to the future. You can now free Manfred and go to the Elemental Skylands.

Key of Courage
Complete the Corrupted Future. It is the unlocked by defeating the Demon King and using the crown in the right side room in the HQ.

There are 6 musical notes to collect to complete the melody and this is the order you should acquire them. Once you've collected them all, you can interact with the music box to unlock this achievement and move on to the final boss fight. I don't need to go into detail on how to acquire them because their locations are obvious enough with the hints from The Prophet and you can buy the shop upgrades to reveal hidden passages/power seals in each zone which will lead you to the notes at the same time.

Autumn Hills (Key of Hope)
Searing Crags (Key of Strength)
Sunken Shrine (Key of Love)
Underworld (Key of Chaos)
Elemental Skylands (Key of Symbiosis)
Corrupted Future (Key of Courage)

Here's the completed melody.