To get the achievement; you need to collect 21 Aurora pieces.
They appear at certain places in the game at random times, and will become visible when you're near them, however they can be difficult to spot especially before they fully materialize.

You can collect multiple pieces on the same spot, so the easiest way I found to get them is flying from the grimoire down and right to the ocean edge.

Possible locations

Left of the grimoire.


Right of the grimoire.


Just under the grimoire.


Bottom of the grimoire, where you perform the first song.


Above the lake.


Left side of the hill just after the lake.


Right to the lake before the hill.


On the hill, right of the lake.


Between the small hill and the portal.


Under the mudhill.


Left of the mudhill.


Right side of the mudhill, just before the ocean.


At the Ocean


At the Ocean, Day

Contact me if there's anything and good luck.
Got the tree of dreams 2 times, but there are probably locations I missed.

Enjoy.
Fera