The captured merchant has revealed key information, allowing you to narrow the field of suspects of who could be the Ivy in the Immortal Flames.
This quest is available if you have completed the quest For the Greater Good.
Starting the Quest
Talk to Alphinaud in the Observatorium in the Coerthas Central Highlands to begin the quest. There will be a cutscene.
Speak with Alphinaud
Go to Gridania. You can get there by going southwest to the path to the North Shroud, then go east from there to enter Gridania. From there, you can go south to Nophica's Altar.
Or if you teleport to Gridania instead, you can use an aetheryte there to go to the Conjurers' Guild.
Once there, talk to Alphinaud.
Speak with the Silent Conjurer
Talk to the Silent Conjurer and say Yes to proceed to the Lotus Stand. There will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.
Speak with Alphinaud Again
Talk to Alphinaud to proceed.
Speak with Ilberd
Go northeast to the path to New Gridania. Go east from there to the quest icon on the map. Speak with Ilberd to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Ilberd to accept the next quest: Chasing Ivy. There will be a cutscene.