Demonwing, gentlemen, is pretty much the single most awesome thing to go around the planes of D&D cosmology, or at least the lower ones.
Once upon a time, the great and mighty Demon Prince Demogorgon did a bit of lateral thinking on the nature of the planes, coming to the following conclusions: A) the layers of the Abyss were malleable and could be shaped into any form their inhabitant liked, and B) there were infinite layers (presumably disregarding those sources that claim there were precisely 666 of them), so who would ever even notice if one of them went missing?
So what did he do with this newfound insight? He gathered around some demonic smiths and wrights and commissioned a motherfucking sailing ship, made out of an entire abyssal layer. Because if you thought you hated Abyss enough already, imagine one of its infinite (or 666) layers literally sailing the seas to attack your hometown.
Demogorgon appointed a balor named Straoth to captain this new ship, and though he probably found it one of the coolest gigs in the multiverse, he was less cool about being Demogorgon's lackey, and after several centuries began to plot. He turned the ship over to his subordinate (a glabrezu named Ungurth) and himself descended down into the "below decks", so to speak, where he began to forge weapons and establish alliances and contacts, all in an effort to free himself from Demogorgon's will and allow him to take this motherfucking ship wherever he wanted.
Unfortunately, while he did so, Demogorgon himself sold the ship to a mortal wizard named Emirikol, who in turn eventually passed it over to a group of pesky mortal adventurers. Because what better way to put a spanner in the gears of a demon than involve these shithead murderhobos?
Manned by Ungruth and thirty spectral trolls, Demonwing unerringly navigates the River Styx, going wherever it's ordered to go. Unfortunately, Ungurth doesn't take commands from some petty mortals, only listening to his superiors (nalfeshnees, mariliths, or balors like Straoth), so if a player party is on board, they better know where it's going because there's no changing that. The ship's made out of stone. Trying to get on board by any other means than the gangplank results in a shock of 12d6 damage lightning before being bounced off by a wall of force.
The lower decks, as with any other "regular" planar layer, are infinite. It looks less like a ship here and more like some underground construction carved into solid rock. There's no penetrating the hull: if some poor bastard decides to dig through, he gets to dig forever because there's nothing but rock there. Anyone living down here will consider adventurers and tourists as intruders, and will react accordingly.