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 isn't done conventionally; you must discover a unique method. Explore the ship, interact with NPCs, and gather clues. The key is accessing Magister Dallis' cabin and obtaining the Ancient Empire Songbook.
Search the bodies of the deceased magisters and gheists on deck. A Sodden Diary on one corpse contains the password to a stateroom door. A skill check on the northern stateroom door might also reveal the password. You'll also need a Strange Gem, found later, to open the door. A magic mirror nearby allows respeccing characters.
Using the password from the diary (or skill check), and the Strange Gem (obtained from Bishop Alexander's regalia after interacting with him), unlock the southern stateroom door using the password "Fortitude." This leads to Magister Dallis' cabin. This cabin also contains a hidden hatch with gheists and a Teleportation Prism.
Inside Dallis' cabin, speak with Tarquin and Dallis, then 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 interaction will be limited once the ship sails.
Inform Malady of the songbook. Go to the west side of the deck, interact with the dragon figurine, and choose to sing the song. The Lady Vengeance will begin to move. Be prepared for an immediate ambush from powerful magisters. Ensure your party is adequately prepared for a challenging battle.