This mission is unlocked after Chapter 7.
To start this quest go to the second floor on Ariela's Tavern and talk to the Dye Maker. She'll ask you to get four lots of something called Angel Thread, to get it, you'll need to find some very rare blue birds, which are on the Central Plaza.
To get the Angel Thread, you just need to point at the birds in seek mode. They are in the following locations:
- On top of the central obelisk.
- To the west of the fence that surrounds the obelisk.
- Hanging lamp near to the eastern gate.
- Hanging lamp near to the western gate.
The first three will give you the thread in a cutscene, the last will go flying to the Castle Gate Plaza, locate him there, and job done.
After getting the four threads, return to Ariela's Tavern and give the treads to the Dye Maker, she'll reward you by giving you the Incredible Invisibility (not sure if it's the correct name), which allows you to make the upper part of your armor invisible.