Wreychtmirk

Wreychtmirk is described in "A Player's Guide to Law" in Planes of Law. It's a cube in Acheron where the River Styx flows across all six faces. On each face, the Styx flows into a different plane, and each face of the cube somewhat resembles the plane the Styx flows into. The Baator face is orderly but corrupted, the Gehenna face is volcanic, the Abyss side is disorganized and covered in carnivorous Abyssal flora (including viper trees), the Carceri face is swampy and filled with razorvine, the Gray Waste face is desolate and colorless, and the Pandemonium face is rocky and wind-torn. One would assume, anyway. The town of Mesk, incidentally, seems to be on the Baator face.

Throughout most of Acheron, the Styx isn't nearly so neatly bound within trenches exactly 330 feet wide and 330 feet deep, as it is in Wreychtmirk. Instead - although occasionally it'll flow through channels or tunnels - it defies gravity and plummets from cube to cube, sometimes sliding across a cube's face in vast sheets and drowning entire armies, raining down unexpectedly as the cubes shift, and eventually somehow falling between layers to end up as frozen shards of ice in Ocanthus. If you want to travel on the River Styx in Acheron, then, Wreychtmirk is considerably more convenient than throughout most of the plane.

So my first question is this: if the River Styx flows into six different planes on Wreychtmirk, where does it flow from? That is, what happens if the PCs row upstream? Planes of Law says "only the marraenoloths patrolling the Styx know the way," but how useful is that, really? If you need a marraenoloth to get to Wreychtmirk, you might as well ask the marraenoloth to get you to wherever you really want to go, and skip Wreychtmirk entirely. It's not any faster. Besides, if the PCs go upstream past the edge of the cube, they'll end up somewhere, even if they don't know where that is, or how to get there from anywhere else. So what do they see?

I see a few different possibilities.

1. It's impossible to go upstream. The portals are one-way. This strikes me as needlessly frustrating and uninteresting.

2. You end up on a completely random (or effectively random from the PCs' perspective) section of the Styx. This is generally how the Styx seems to work: as the Planescape Campaign Setting said, "The flow isn't linear or predictable, either. On one journey, a boatman goes from Baator straight to the Abyss; on the return, the river shifts its banks and carries the cutter from the Abyss to the furnaces of Carceri. The flow is unmeasured and unmapped." Well, okay, that's fine, except Wreychtmirk is already an exception to this, since its faces reliably lead to the same planes every time. And really, the Styx has to be somewhat predictable for characters who aren't marraenoloths to have any chance of reaching their destination at all. The main advantage to marraenoloths is that if you're with them, amnizu and hydroloths won't harass you, and you won't tip over. Nobody can know all the shortcuts the marraenoloths know that allow them to get anywhere the Styx touches in 1d20 hours, but it ought to be possible to transverse certain well-known routes.

3. Baator. The River Styx, with many, many exceptions, moves from its headwaters in Pandemonium through the Abyss, to Carceri, to the Gray Waste, to Gehenna, to Baator, and finally to Acheron. There are, of course, countless tributaries and offshoots that cut across the planes, and even the mainstem of the river isn't completely linear, but that's the general path. Presumedly the majority of Acheron's Styx-water originates in the Forgotten Lake in Nessus before spilling between planes. So maybe there's a place in Avernus or Stygia where the river splits into six distributaries, each of which flows through a different portal leading to Wreychtmirk's six faces. If so, that'd be an even more important place for planar travel than Wreychtmirk itself, since travelers wouldn't have to trek across the cube to get to the part of the river they want to get to. Maybe the split is right next to Tantlin in Stygia, which would make that city much more significant than it currently is. Then again, any portion of the Styx allows someone who knows the way (mostly marraenoloths) to get to any other lower plane; Wreychtmirk isn't that special in anything but its predictability, and its status as the safest Styx route in Acheron.

4. The branches of the Styx originate on the same plane they arrive at. This honestly makes the most sense to me. If a cube face is Abyss-themed, it's reasonable to assume the river connects to the Abyss at both ends. It might well end up on a different layer than it began at, which would make it important on to those who want to travel on their original plane as well as to those who want to cross planes from Acheron. It could also lead to the exact same spot it began at (for example, the river could move from the Teardrop Palace in Gehenna to Wreychtmirk, then back to the Teardrop Palace again, making it useful for those who want to cross planes, but not useful for those who just want to travel around Gehenna).

The second part of the question is, of course, where in each plane does each branch flow? It could be random, but as most of the planes the Styx touches are infinite, you still might never find your destination at that rate. Unless you have a marraenoloth, in which case you don't even need to pick the appropriate face of the cube: a marraenoloth could get you to the Fateless Harborage on the Plain of Infinite Portals from Gehenna as quickly as it could from any other layer in the Abyss. I like the idea of a more or less stable route, but with a percentage chance of getting lost that increases the more planes and layers you cross.

I'm going to answer my own questions, but this isn't intended to be the final word on the subject. Everyone else should feel free to discuss the subject as well.

Abyss: With viper trees on the cube, one might think the river must connect to Azzagrat, but Azzagrat doesn't have a branch of the Styx (it has the River of Salt instead). This is all arbitrary, but I'm going to suggest it flows out of Goranthis, Socothbenoth's realm, and into the Abyssian Ocean. There should probably be a few different possible entry points, including Wormblood, Gaping Maw, Shendilavri, Durao, and the Plain of Infinite Portals. The fan-created city of A'shad'ifohg (from the Mimir) would be a good connection, since it's built over the Styx and supposed to be a major trading burg.

Carceri: No visit to Carceri is complete without a visit to Mount Othrys, home of the titans. Carceri's a prison, so it wouldn't do to let too many areas get access to other planes. Let the river loop back on itself in this case.

Pandemonium: Though we're repeatedly told the Styx has no source, we're also told that its headwaters are in Pandemonium. Most of the Styx on this plane takes the form of small streams, not very useful to travel on (even the marraenoloths avoid them), so I have no idea what sites and settlements would be good to connect via a detour through Acheron. Actually, if any plane deserves a completely random connection, it's Pandemonium. It might be a decent way for denizens of the Madhouse to get to and from Winter's Hall.

Gehenna: Dragon #358 mentions the Styx flowing through Hopelorn in Krangath, which is odd, since it's supposed to only flow through the first layer. Using Wreychtmirk as a shortcut would make a lot of sense. On the other side, the Teardrop Palace is an important trading destination.

Gray Waste: Wreychtmirk could explain how the River Styx gets into the Underworld in Pluton. On the other side, the river is supposed to come within eyeshot of Khin-Oin.

Baator: I'm going to go with Tantlin, after all, but as an exit point. I'll have it enter near the confluence of the River Styx and the River of Blood in Avernus.