After you thwart the Masked Mage at the Guardian Tree, Kan-E-Senna herself invites you to be Emissary at the Greenbliss ceremony.
This quest is only available if you started the game in Gridania, and you have completed the quest To Guard a Guardian.
Starting the Quest
Talk to Lewin in the Seat of the First Bow (accessible in the back of the Archers' Guild) to start the quest.
Check on the exit to leave the Seat of the First Bow, then go to the Carline Canopy south of the main Gridania aetheryte and talk to Miounne.
Collect the Artifact
Leave the Carline Canopy and go north, then northwest to the Carpenters' Guild. Go inside and you will find Beatin in the back of the main room. Talk to him to get the wooden lockbox.
Deliver the Lockbox
Go back to Miounne and hand over the wooden lockbox.
Next Main Scenario Quest
After completing the quest, talk to Miounne to accept the next quest: Renewing the Covenant.