This Divinity: Original Sin 2 guide details how to get the Lady Vengeance sailing. After escaping Fort Joy and removing your Source collar, your goal is to set sail, but this requires unconventional methods. The solution involves exploration, interacting with NPCs, and solving a puzzle.
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Investigate Dead Bodies
Begin by searching the bodies of the magisters and ghosts on deck. A Sodden Diary on one corpse provides a password. You can also obtain the password via a skill check at the northern Stateroom door. Crucially, you'll also need a Strange Gem, found later. A magic mirror nearby allows respeccing characters.
Find the Portside Stateroom Door
Using the password from the diary (or skill check), access the ship's quarters. Interact with the unconscious Bishop Alexander to find the Strange Gem. Use this gem and the password to unlock the southern Stateroom door. This leads to Magister Dallis' cabin. The cabin also contains a hidden hatch with two ghosts and a Teleportation Prism.
Find the Ancient Empire Songbook
Inside Dallis' cabin, talk to Tarquin and Dallis. The Ancient Empire Songbook is on a pedestal. Reading it reveals the song needed to move the ship. It's advisable to speak to all NPCs before proceeding, as interaction will be limited once the ship sails.
Getting the Ship Moving
Return to the deck and speak to Malady. She'll ask you to sing the song. Find the dragon figurine to the west and sing the song. The Lady Vengeance will begin to sail. Be prepared for a challenging battle with magisters immediately afterward; ensure your party is well-equipped.