The pixies won't let you go free until you have completed some chores for them, and Sul Uin has one such task.

Requirements

This quest is available if you have completed the quest Il Mheg, the Faerie Kingdom.

Starting the Quest

Talk to Sul Uin in Il Mheg to begin the quest.

Sow Everbloom Seeds

At each of the quest icons on the map, dismount and target the sparkling Destination, and then you can either examine or target it and use the Everbloom Seeds on it from the Key Items section of your inventory, or you can target it and use the icon to the right of the quest name in your list of active quests.

Speak with Sul Uin

Go to the quest icon on the map and talk to Sul Uin to complete the quest.

Next Main Scenario Quests

After you complete the quest, if you have also completed Ys Iala's Errand and Oul Sigun's Plea, talk to Thancred to accept the next quest: Unto the Truth. There will be a cutscene.