Acheron (PWH)

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Battle. That's all there is on Acheron—conflict, war, strife, and struggle. For more than one reason, many planewalkers associate this plane with the clang and clash of metal against metal. Partially, this comes from the eternal striking of sword against shield, and blade against blade. It also comes from the sound of two of the gigantic iron cubes that make up this plane smashing
against each other.
Each of Acheron's four layers consists of these cubes, which float through an infinite, air-filled void. Some of the cubes are small, but others are huge enough to hold whole cities and kingdoms. Each face of a cube is habitable, and gravity is always directed toward the center of a cube. (Not all of the cubes are actually cube-shaped—some are plates and other geometric shapes, but since most are cubes they're all called by the same name.)
Worse, the cubes constantly move and occasionally collide, ringing a toll of disaster for anyone standing on one of the clashing cubefaces. The cubes are also riddled with tunnels and caverns, providing more space to marshall forces and fight battles. Travel between cubes is possible through use of special gates and spells.
Acheron is the home of the goblin and orc deities, as well as similar evilly militaristic types. These powers send their petitioners into continual battles for dominance. An Acheron petitioner cares little for form, only function. The beauty of a weapon matters little—what counts is only how badly it wounds a foe that it strikes. While skill isn't completely overlooked, in the end, brute force is king.
The first layer of the plane, Avalas, is the most heavily populated, but it's also where most of the fighting goes on. Yugoloths, baatezu, modrons, dwarven einheriar, petitioners (folks who died in unglorious battle), and planar mercenaries do battle with the aforementioned humanoids as well as with each other. There're towns and fortresses throughout, but mostly it's a layer of battlefields.
The second layer of Acheron, Thuldanin, is of more interest to canny planewalkers. It's the final resting place for more weapons and more types of weapons than a single cutter is ever going to know of or see. Machines and devices of war from endless conflicts throughout all the planes, including the Prime, end up here. Scavenging for intact weapons can be a valuable undertaking, since many of great quality, fantastic power, and intriguing mechanisms can still be found and used, or at least copied. A word of warning, however—the dark of this layer is that anything (including equipment, weapons, and flesh) staying too long turns to stone.
Tintibulus and Ocanthus, the third and fourth layers of Acheron, aren't much visited. Folks who frequent such places aren't the types to rattle their bone-boxes without some significant garnish, so the chant remains dark. A few say that a huge city on Ocanthus is home to a powerful, isolated race, but this has never been confirmed. 
Tarsheva Recommends: A cambion named Thrao sneaks about the plane looking for magical weapons. Chant is, he's got one of the greatest hoards on the Lower Planes (mostly weapons of low power, but he makes up for it in quantity). His prices are high, but his selection is superb—and he has a number of odd magical weapons, like garrotes, pikes, blowguns, arquebuses, and more.