To earn the Stormculler trophy in Assassin's Creed Odyssey you must purchase the second episode of the Legacy of the First Blade expansion (or own the Season Pass), download and install it. You must also start the second main quest of the DLC, Into the Storm. During the course of this quest you will infiltrate a fort. You'll see an orange diamond marker in the fort which leads you to interact with a scroll. This starts the quest On The Grapevine for the Tempest, which leads to some clues to Order members too. There are only four to find in this DLC beyond The Tempest. Finish Into the Storm to officially unlock the new tree of Ancients in your menu.
1. Mekakreon the Unbreakable
This guy turns up during the main quest "Theatrics and Espionage", and you have to kill him to complete the quest.
2. Phila the Tempest
You will defeat the Tempest one way or the other during the main quest. Although there are decisions you can make in the dialogue that affect the exact ending for The Tempest, you can't actually avoid the fight.
3. Nestor the Formidable
Clue: Olouros Fortress, Scavenger's Coast, Achaia
Follow the On The Grapevine quest to this Fortress. It's not the quest marker itself you need, but a scroll on a desk in the same room as the Nation Chest.
Nestor is out to sea, sailing around between the northern coast of Achaia and the western coast of Elis.
4. Augos the All-Seeing
Clue: Defeat Nestor the Formidable above
Augos is alone by the docks on the Scavenger Coast in Achaia, directly north of the Statue of Poseidon fast travel point.
5. Sophos the Broker
Clue: This is very close to Augos above, at the Shipwreck Cove location just east along the coast. The Statue of Poseidon is still the closest fast travel point. The clue is on a dead body on the broken ship in the middle of the rock formation off the coat.
Sophos himself is in the village of Patrai in the Mount Tanachaikos region of Achaia. Fast travel to the Temple of Zeus and find the Ancient near the docks.
Once you have defeated all of the Order members in this DLC you will unlock the trophy.