A massive and magnificent haunted boat, the Shadow is possessed by the Lady of the Shadow, and the souls bound to it that form it's crew. But if the crew should be defeated, a creature can attune to the boat once more. 

Captain's Blessing The attuned creature is now considered the captain of the vessel, which will only sail for it's captain. 

Dictate The Storm While a creature is attuned to the boat and at it's helm, the creature may use a bonus action on each of their turns to determine the weather in a one mile radius of the Shadow. 

Whispers of the Lady As an action, the creature can allow the whispers of the Lady of the Shadow into their and the crew's minds. For one hour, all effected creatures have advantage on attack rolls, non-death saving throws, and skill checks, but automatically fail all death saves they make. 

Curse of the Shadow If the Captain dies, the rest of their crew immediately dies as well, the rigging turning against them and hanging all who live in a moments notice. The souls of creatures who die this way are trapped, and the crew's statblocks get replaced with the Crew of the Shadow statblock, while the Captain's statblock gets replaced with the Captain of the shadow statblock. 

Notes: someone who's bested the Shadow's crew