The Why or Hook:
The hook is simple, the PCs already work for the guy who sends them on their missions and they get all they need as PCs and players.
The Where:
A Pirate Ship on the open seas, sailing between islands along the coast of a lush tropic continent.
The Who:
Black Ships. Massive fiend-constructs used by local fiends to capture local souls and slaves. These things are essentially usd by powerful fiend lords as planes-hopping pleasure craft, travelling the planes and sampling the local fare.
They're also loaded with treasure. Fiend-tresure from all over the place.
The How:
They're 7th lvl PCs, with the Wish economy and will be working with a crew of mercenaries (pirates) their boss knows.
Something I wrote in in my humble opinion regarding the mounted combat thread.
I so have to make a character who is ridden by their cohort.
A druid who stays in one form al the time and then is ridden into battle by their lower lvl 'beat-stick' cohort.
I think that the Black Ship captain in the adventure I'm writing will have a druid who stays wild-shaped as a parrot, then casts a pair of animal-growing spells or wildshapes into a larger flying animal. The druid is the actual NPC, the pirate on it's back that's using an auto-loading musket is the cohort.
I don't know if it should be the villan or the PCs transportation officer. Since they're working with pirates to attack this particular Black Ship; which is manned by pirates, but they're undead/fiends/monsters.
The ... what?
The ship the PCs is on will be crewed by anthropomorphic sea creatures; squid, octopus, albatross, tuna, shark, dolphin and... maybe a anthro blue whale (who's the cook, and he only knows how to make "Krill Soup").
Anyway, this is mostly to trawl for ideas and help me organize this idea out.