Avalas, the Battleground, is the first layer of Acheron, and the one most associated with the plane; most visitors don't travel any deeper than Avalas, after all, as to a visitor there's little lower they need. This is the plane of active conflict, of ongoing war, of the midst of battle. It's here that the massive clashes most associated with Acheron — orc against goblin, baatezu against tanar'ri, rakshasa against all — all occur. This is the most lively of the four layers of Acheron by far, and ironically the most livable.
As the only layer which supports native flora and fauna, Avalas holds the greatest population of all the layers of Acheron. Numerous cities and townships dot the cubes, some caught in the midst of war, others metering Acheron's natural proclivity towards combat into sport or duel. From small villages like Mesk to great fortress cities like Vorkehan, there are many that call Avalas home.
Most of the inhabited sections of Avalas is divided into multi-cube states, and while many of the cubes are embroiled in warfare, many more are relatively free of open conflict; as the plane of orderly conflict, there tends to be little in the way of strikes deep within a state's territory, as such actions are seen as contrary to the Rules of War. Travel within Avalas, as a result, can be relatively safe so long as one keeps away from the various fronts. Most significant states in Avalas are planar-held and tend to span a half-dozen cubes or so, but the Rakshasa empire is said to span dozens hidden under their illusions, and the orcish and goblinoid pantheons both hold at least a dozen apiece beyond the fronts of Clangor and Nishrek. There are also some rumored cubes held by yugoloth, but of course that's not likely to ever ascend above the level of rumor.
However, one must always be wary for the various rogue armies of Avalas, those forces whose states have been destroyed and which are now able to sustain themselves only on what they can claim from others. Still obeying the Rules of War, they have not been declared losers by the plane, but they struggle to survive, and that struggle can make them even more willing than most to edge right up to the limits of what is permitted in Acheronian War Made Raw. Most rogue armies are made up of undead or constructs, as few mortal armies can manage to maintain the supplies necessary to satisfy the upkeep of their troops without the support of a state, but there are some few exceptions.
Rogue armies of note include the mummified troops of a long-forgotten emperor-saint of the kuo-toa; the varied undead forces of the lich Boretti, the self-proclaimed Necromancer-King; and the House of Reddirk, an ancient branch of the bladelings who divided from the majority of their people ages back, before the settlement of Zoronor. And of course, one must consider the hassitorium as rogue armies in and of themselves. Ancient vehicles created by long-since extinct natives of Acheron, the hassitor, eight of these mobile fortress-cities still exist in functioning condition, wandering Avalas under the control of local forces who use their power to fend off any that would seek it for their own.
- Dragon #358 - Savage Tidings: The River Styx, pg.69
- Manual of the Planes (3e), pp.123-125
- Planar Handbook, pg.183
- Planes of Law - Acheron, pgs.9-10,12-20
- Planes of Law - A Player's Guide to Law, pgs.10,12-13