It is a dusty rune. That should have been their first clue that it was dangerous.
Perhaps the product of another universe or time, this vessel appears as an early Tall ship, but is made of iron from keel to spar. The sails are a battered canvas but are nearly indestructible, but in truth do little to move the ship - the ship is able to move even with no winds.
The ship is powered by life force, requiring regular sacrifices to ‘charge’. The victims are cast in front of the vessel as it moves, it’s razor sharp bow bisecting them. The ship may initially be missing key components and the ritual to power it would also require research by would-be users.
The ship has no built in weapons aside from the bow, which can easily cut even large wooden ships in twain.