Black Ships are even more sinister than Ghost Ships.
Long ago the Fiends taught the Orcs how to create this despicable vessels and so they’re often called Orc Ships. It’s not a well guarded secret, since Fiends and Orcs crave to spread corruption and decay, but few are those, even among the evil, who dare to mingle in this sort of shipcraft.
Black Ships can sail almost anywhere, provided the right price is payed. These almost always big and heavy ships have a big furnace at the center of their lower deck where “suitable fuel” is burnt, so to make the barge float in whatever non-watery environment it’s navigating. “Suitable fuel” means something magical enough to make another Ship float in that particular Tide, Stream or Surge. Quality of the material is extremely relevant: for example, burning trees grown in a Stream of Emotion will make the Black Ship float for a short time in that Stream, but burning the carved wood of an Halfling Ship suitable to sail Emotion Streams would let the barge navigate longer and smoother. Working on this principle the Orcs soon discovered that burning Elves attuned to a specific Tide would make the ship run longer and better on that Tide than burning (just) their Tuned Ships, while burning Halflings of the Docks makes the Black Ship suitable to sail on any Stream for a brief time (since they’re small), and Kobolds make a very suitable fuel for Surge sailing (especially since they’re drops and, being Dragons Scales, they burn very slowly). Though it’s hard to prove, Orcish Shamans claim that burning the captives alive provide better fueling than burning their corpses. It’s proved that Orcs like it better.
The combustion releases a dark dense smoke that sticks to the Ship and its course, darkening the wood and bone and rusting the iron. Where Black Ships pass vegetation withers, the water is poisoned and acid rains may fall. It’s also rumored that places where a Black Ship passed are more likely to attract evil people and creatures, not to mention natural disasters and wars.
Of course moving these Ships outside water is extremely demanding, since the “fuel” is often hard to acquire and limited in availability, so the Orcs don’t move them often (praise the Lifebringer!) or without some strong (and evil) motivation.
Raids to acquire and stock any sorts of “combustibles” are long established tradition among the Orcs, who call it “Harvest the Fodder”.
Feeding the furnace is also not free of risks: Orcs propelling their Ship with undeads to have it sail as a Ghost Ship found out (too late) that the misty smoke it exhaled was turning them into undeads themselves; Black Ships sailing too long on the same Tide, Stream or Surge, shipwrecked due to a sudden and powerful adjustment of that powerful medium (after all they are pure essences).
Feeding the furnace is also known to increase the sailors bloodlust, that must be frequently satisfied or they will turn to one another.
Because of the devastation they spread, Black Ships are usually attacked on sight by almost any fleet on Shore.
The furnace doesn’t give the Black Ship any propulsion: orcs usually use slaves chained to the oars and whipped to exhaustion, but other means are sometimes employed.