The Nocturne is the Nocturnal's Ship.
It is the ship that Grace Tempest is rescued by when she is drowning in the first book "The Demons of the Ocean". It is a strange ship and there are still many more mysteries about it. One of the strange things is its figurehead, a female vampirate named Darcy Floatsom. Both of the Tempest twins have visited it.
It appears to be quite an organised ship with a feast once a week food for the donors and the donor's blood for the vampires.