This guide details how to get the Elven ship, Lady Vengeance, sailing in Divinity: Original Sin 2 after escaping Fort Joy. The process involves a series of puzzles and interactions.
After removing your Source collar, your goal is to get the Lady Vengeance underway. This requires unconventional methods; you can't simply use the helm. Exploration, conversation with NPCs, and puzzle-solving are key. The ultimate objective is to access Magister Dallis' cabin and obtain the Ancient Empire Songbook.
Begin by examining the bodies of the magisters and ghosts on deck. Loot them for clues and items. A Sodden Diary, found on a magister's corpse, contains a password needed later. You can also obtain the password by successfully completing a skill check with the northern stateroom door. Additionally, locate a Strange Gem; it's required to interact with a specific door. Note that a magic mirror nearby allows for respeccing characters.
Using the password from the diary (or skill check), access the ship's quarters. Interact with the unconscious Bishop Alexander to find a Strange Gem within his regalia. Then, use the gem and the password ("Fortitude") to unlock the southern stateroom door. This leads to Magister Dallis' cabin. This cabin also contains a hidden hatch with two dangerous ghosts and a Teleportation Prism.
Inside Dallis' cabin, speak with Tarquin and Dallis, exhausting their dialogue options. Locate the Ancient Empire Songbook on a pedestal. Reading it reveals the song needed to move the ship. It's advisable to speak to all NPCs before proceeding, as access will be limited once the ship sails.
Return to the deck and inform Malady about the songbook. She'll ask you to sing to the ship. Find the dragon figurine to the west of the deck and select the option to sing. The Lady Vengeance will begin moving. Be prepared for a challenging battle with powerful magisters immediately afterward. Ensure your party is adequately prepared.