After Miounne reads Luquelot's letter, she has an urgent mission for you to undertake.
This quest is only available if you started the game in Gridania, and you have completed the quest Dread Is in the Air.
Starting the Quest
Talk to Miounne in Gridania to start the quest. There will be a cutscene.
Deliver Miounne's Letter
Bowlord Lewin is in the Archers' Guild. If you have attuned to the Archers' Guild aetheryte, you can use the main aetheryte to get there. Otherwise, you can get there by going north from the Carline Canopy, then east, then southeast. It will be at the end of the path.
In the Archers' Guild building, go through the arched doorway to the left of the reception desk, then go along the left wall and up the stairs at the end, then go to the right and talk to the Gods' Quiver Bow. Say Yes when asked to proceed to the Seat of the First Bow. Once inside, talk to Lewin, and hand over Miounne's Letter. There will be a cutscene.
Make Haste to the Guardian Tree
The exit is behind you, so check on it to leave the Seat of the First Bow. The Guardian Tree is in the Central Shroud. If you have attuned to all aetherytes in Gridania, you can use the aetheryte outside of the Archers' Guild to go to the Central Shroud. Otherwise, go west from the Archers' Guild, then when you reach the main aetheryte of Gridania, go south, then southeast to go through the exit to the Central Shroud.
Once in the Central Shroud, make your way southwest, going past the large tree root (not up it), to the stream beyond. Go along the stream and you will see the Guardian Tree. Before you reach the tree, look for the Gods' Quiver Instructor. There will be a glow around him, indicating another instanced battle. Check on him and there will be a cutscene, and then the battle will ensue.
Solo Duty: To Guard a Guardian
A number of Ixali Lostwings and Ixali Slowbeaks will attack, as well as Quzal Huatotl the Shrieker. Ignore Quzal for now and focus on slaying the Ixali. After a while, more Ixali will appear, including two Ixali Sincallers that target you specifically. First, kill the Sincallers, then focus your attacks on the enemies on the left, before killing any that remain on the right. Then attakc Quzal until a Blanga appears. Focus your attacks on the Blanga, then when it dies, attack Quzal until he is defeated.
There will be a cutscene, and then another battle will take place. Attack the Lesser Gargoyle. Eventually Yda and Papalymo will appear and assist you in the battle. After the Lesser Gargoyle dies, attack the Masked Mage until he is defeated.
There will be another cutscene. This one is important, so you might want to pay attention. You can go to an inn room and use The Unending Journey to watch it again if you wish.
Speak with Lewin
Go back to Gridania, into the Archers' Guild building, and make your way to the hallway in the back as before. Talk to the Gods' Quiver Bow and say Yes to enter the Seat of the First Bow again. Talk to Lewin and there will be a cutscene. Afterward, you receive a Gridanian Ring, which is exclusive to players who started in Gridania.
Next Main Scenario Quest
After completing the quest, talk to Lewin to accept the next quest: Festive Endeavors.