Edit: ::Shakes fist at Teneb for being so fast)
Apparently the hint was about the Vampire Coast with four legendary lords, including Luthor Harkon, and a new army list as well as new mechanics.
Link to a blog post with more details.Objectives
The great Merwyrm Amanar has been awoken from its slumber by the vacillations of the Great Vortex. It terrorizes the sea lanes, summoning vast storms and attacking coastal ports on a whim. Only one who wields the Star-Metal Harpoon will tame the beast and become immune to its ravenous hungers. However, the Harpoon must first be found, and invigorated using the three verses of a secret, ancient sea-shanty.
Each playable Legendary Lord must grow their dread reputation to become the most Infamous pirate on the seas. Certain opposing Pirate Captains know the three verses of the ancient shanty and, as your Infamy rivals theirs, they will seek you out in attempt to destroy you. If you best each in battle, you’ll learn each of these verses, all three of which are required to magically charge the Star-Metal Harpoon.
Only with the fully-powered Star-Metal Harpoon in hand can you embark on the final, fateful battle, bend Amanar to your will, and become the greatest sea-scourge the world has ever seen!
The Curse of the Vampire Coast Campaign Pack contains:
4 new Legendary Lords
New Lore of Magic: The Lore of the Deep
New roster: command towering constructs, ghastly pincered sea-beasts, ghoulish horrors and musket-wielding piratical zombie hordes in battle!
A suite of all-new campaign mechanics:
Treasure Maps: Find treasure maps and dig for booty
Ship Building: Upgrade each Legendary Lord’s unique ship
Infamy: Rule the waves to become the most infamous pirate of all
Pirate Coves: Establish hidden enclaves in your enemies settlements
Fleet Offices: Reward your undead Admirals with positions of power to improve their loyalty
Pieces of Eight: Sail the seas to unlock eight new Regiments of Renown
New battle mechanics:
More Powder: Use your Heroes to keep your gun-lines topped up with ammo
Extra Powder: While black powder is plentiful, each unit will benefit from a variety of missile-related buffs
Important to note though, as they mention in the F.A.Q, Creative Assembly are not implementing naval battles with the DLC. There will be, however, be battles you can fight instead of auto-resolve. One is the battle takes place on a nearby island (As they mention, very similar to a mod that was recently released) and the second will be battles against Black Arks will take place on a map specific for them (A nice addition).
One of the new mechanics caught my eye. The hidden pirate cove/enclave sounds like it would work well for Skaven establishing underempire outposts in enemy cities.
The unit list seems very interesting, with huge crabs, giant crabs, that horrible mouthy ghoul thing, multi-cannon wielding ship constructs and pirate zombies. There's also a new lore of magic, lore of the deep, hunting for treasure using maps, gaining infamy for your lords and upgradable main ships the lords have as well (I assume kind of like the Black Arks).
Along with the DLC will come some other updates as well. The Vampire Counts will apparently get a reworking of their mechanics and abilities. There is also a FLC lord coming for the Mortal Empire and Vortex campaign.