Key Object

There is only one key object so far :
The Circle :
To create the circle, you need to find the tome on the last upgrade of Sky Islands, to be able to craft it in the Amplifier building.
You'll need to collect all 4 parts of the ring, by completing the 4 supply objectives of Sky Islands.
Once crafted, you can sleep in order to trigger the endgame credits.
There is one letter coming after reloading a completed savegame. Also, sleeping again won't trigger the credits again.