The material plane is infinite, but teleportation is rare; the map above shows only the ‘known world’ as mapped in Ileoja.
Vessar: a region largely of plains. Nominally the territory of the Ileojan republic.
Norselands: The primarily dwarven city of Eldheim is second only to Ileoja. Eldheim and its protectorate are at war with an alliance of cities of the Eastern Norselands, centered around Langt
Eastwood:
Glavnazh: a great desert. Its interior forms the boundary of the known world, though according to its few inhabitants there is another sea beyond.
Adamant Sea: The larger of the two known seas, but the less dangerous. Across it to the east there is said to lie another continent, though little about it is widely known.
Nainland: Dominated by the marshlands around Lake Shimmer, Nainland maintains a declining monarchy, aided by a tenuous alliance with the local fey, who are less reclusive than most. Although the kingdom of Nainland is larger in territory than any other state on the continent,
Argent Sea: The smaller sea, but the favoured resting place of krakens. Ships to and from Nera seldom venture too far from the coast.
Alfheim: Homeland of the Svartalfar, the distant ancestors of all other elven races. The Varen elves do not permit settlement or immigration to their territory on the Northeast coast of Alfheim; thus its southern reaches are largely unknown. Trade between Vessar, Ordin, and Varen thrives, however: the mines of Alfheim are rich with mithril and galvorn.
Ileoja: A small city, though among the largest on the material plane. Ileoja is situated within an area warded by the Makers, and so is safe enough for a large population to settle in.
The Maker's wards only allow creatures with a specific arcane mark to enter, and prevent planar travel.
Common: All drow speak Common, as do the people of Vessar. It's a common second language throughout the Norselands, Nain, and Glav Nazh.
Abyssal: A nonmagical derivative of the Dark Speech. All demons instinctively know abyssal, and it's also spoken by many other denizens of the lower planes.
Aquan, Auran, Ignan, Terran: All creatures with an elemental subtype instinctively know the associated languag. The elemental languages are also spoken by many other denizens of the inner planes.
Celestial, Infernal, Modron: Like Abyssal, but for LG & NG, LE, and LN, respectively.
Ildaz: Spoken in Northern Vessar. A major language in ancient times, but has been mostly replaced by Common. Noteworthy for being the native language of several deities.
Ordmil: The language of Ordin. Dialects are spoke elsewhere on the Adamant Sea, and as a second language among the merpeople of the region.
Norse: What the name says.
Nain, Domnian: The Languages of North and South Nainland, respectively.
Probably some other ones: but they're not important enough to write about at the moment. Minor local ones, and ones of other planes or regions that aren't major enough to make up for their remoteness.
Dragons and Fey do not have a language of their own, but, like most long-lived creatures, generally speak most major languages.
Once, the Makers protected the sentient races they had create, but five thousand years ago, they vanished from the world. As their great wards have faded, the world has grown more dangerous. The boundaries between planes have become thinner and portals less predictable; many powerful monsters inhabit the multiverse, and they are not likely to help an inadvertent planar traveler. The uncontrolled creations of the Makers are as likely to be dangerous as benevolent.
Humans fear (for instance) being enslaved by formians, who in turn dread being drafted into the blood war, and even balors and pit fiends fear the epic monsters of some demiplanes. However, travel has its advantages, as even the Plane of Erosion has valuable resources and benevolent extraplanar beings. Occasionally, a traveller can even find an intact magic item from the time of the Makers.
There's definitely societies, but they're small and isolated from each other.
The things people are terrified of would actually be the main use for social skills (for an adventurer), because the outsiders don't tend to kill humanoids. We're too valuable for that. They'd rather enslave you, or keep you as a source to drain energy from, or send you to get something from an area warded against evil by the Makers.
Cosmology: all the standard planes exist, except for the positive and negative energy planes. Instead of those, there are 2 new groups of planes:
The positive planes:The positive planes:
Prolifar (the plane of growth)
Simulcron (the plane of knowledge)
Syllogria ()
Altipros ()
Ettek (the plane of solidity)
Etrivon(the plane of intensity)
Taea (the plane of clarity)
The Maze
The negative planes:
Ukrolus (the plane of impermanence)
Okelys (the plane of shards)
Vodremk (the plane of weight)
Asculos (the plane of fear)
Yrst (the plane of fury)
Itanat (the plane of forgetting)
Reyennon (the plane of sorrow)
The Maze
(Names thanks to SirPercival)
The Maze:
The Maze is the home of the Fey, a plane of enchanted wilderness and strange magic. It takes it's name from the complex and slowly changing shape of space, and sometimes time, which renders it almost impossible to navigate for those unaccustomed to it's nature. It is is a bi-layered plane, in which all reflective surfaces for gateways between the Seelie side, which is minor positive-dominant in many places and co-existent with all the positive planes, and the Unseelie side, negative-dominant and co-existent with all negative planes. It is also are co-existent with the shadow, material, and astral planes.
Ukrolus:
Ukrolus, the plane of impermanence, is unlike many of the negative planes in that there it hosts an abundance of native life, although it is no less dangerous to living creatures from elsewhere. On Ukrolus, the durations of most spells and effects are halved, and those that would be permanent elsewhere may fade after 1d10 years. Creatures gain fast healing 2, offsetting the pervasive negative energy, but age thrice as fast, and must eat, drink and sleep thrice as often. This makes it dangerous to living creatures of the other planes, yet the volcanic islands, atolls and salt-spires that form from it's shallow seas are home to a diversity of insects and plants adapted to the plane, as well as undead who who can resist the plane's erosion of the enchantments that power them.
Okelys:
Okelys, the plane of shards,
[description will be added later]
Vodremk:
Vodremk, the plane of weight,
[description will be added later]
Asculos:
Asculos, the plane of fear,
[description will be added later]
Yrst:
Unlike Asculos, Yrst, the plane of fury, does not magically inspire the emotion it is named for. Instead, Yrst rewards anger, allowing living creatures that feel it to be free of the plane’s negative energy, and granting more negative energy to undead that satisfy the plane’s nature with their rage, empowering them to survive Yrsts’s vicious cold and the hostility of its other residents, kingdoms of lesser undead ruled by the Lavawights.
Itanat:
Itanat, the plane of forgetting, is shrouded in fog, which seeps up from the wells and crevasses that cut through it’s terrain, and gathers around what few creatures dwell upon it’s surface. The fog is a side effect of the plane’s characteristic trait: creatures must make a will save each minute or gain a negative level as their memories evaporate, joining the fog of the plane. This effects even undead and others immune to mind-affecting effects (though they gain a bonus on the save) but the plane's traits can be mitigated by staying where the fog is thick and the air is already saturated with lost thoughts. It’s said the sometimes, undead who tire of existence, yet fear to move on, go to Itanat, to rest and forget.
Reyennon:
Reyennon, the plane of sorrow, has a strange effect on sapient creatures - visitors to the plane find a separation between themselves and the world, as their body is on one layer of Reyennon, with all things material, and their spirit on the other, surrounded by visions of regret. Reyennon does not break the link between soul and body, but makes it distant, rather than close, and choices and sensations take time or effort to pass from one side to the other. Reyennon is where the evil gods claimed negative energy as part of their dominion, and both Vecna and Orcus are said to have had strongholds there when they were mortal.
The curtains of the world:
A shimmering transparent veil thousands of kilometers long and higher than even a dragon can fly lies over the oceans between Alfheim and Vessar, and East to West between the Norselands and the Frostfell. It is intangible, and causes no harm to those passing through it, but blocks most spells and effects, including teleportation.
The Makers:
Ancient beings, no longer present in the world. Their power surpassed even that of the gods, and they created many new species of living creatures, for reasons no longer known. Many of their creations and artifacts remain, and some say all the humanoid races can be counted among those. Some say that they created the world itself. Few records of their civilization survive, and those ancient creatures that remember that era are not inclined to spread their knowledge.
Deities:
Minor gods can appear spontaneously from the material of the astral plane. Most major ones are mortals who ascended during the time of the Makers. (This includes the Greyhawk pantheon listed in the DMG and a pantheon based off of Norse mythology.
Unoriginal, I know, but not creating a whole new pantheon makes things easier for everyone, and Loptr was worth including Norse myth expys for.)
There were once 8 overdeities, but they do not grant spells, and have not been worshipped for so long that even their names have been forgotten by most.
The first heroes:
In the time of the makers, there was no need for great warriors, healers, or mages. Those who were the first to master the skills humanoids need to survive in the multiverse on their own are known as the first heroes. Most mortal disciplines of power were invented by them, and the shape of the world today shows their influence in every facet of life.
Among them are such legendary figures as Reshar and Mordenkainen, but also many less famous yet equally significant pioneers.
The Fey:
The Seelie and Unseelie courts (NG and NE aligned, respectively) reside in the two sides of the Maze, ruling many lesser courts and kingdoms of Fey according to rules often incomprehensible to mortals.
They value and honor the company of extraordinary humanoids - or other creatures - and compete to impress the queen with their guests, yet can be as cold and deadly as any Norseland blizzard.
Few fates, even among the dangers of the planes, are more feared than the Wild Hunt.
The Humanoid Races:
The world is older than history. Even the gods are young in comparison, though they may know something of it. The humanoid races, however, are known to have been created by the Makers, only a few centuries before their fall, with a few exceptions such as the Drow, the first generation of whom were elven followers of Lolth transformed upon her ascension.
The first generation of humans were created somewhere in Ves (the continent the map is centered on), and the elves and dwarves can likewise trace their ancestry back to Alfheim and the Frostfell respectively, though over the millenia since members of each species have migrated and diverged, and all three races are common in most regions. Orcs are said have crossed Glav Nazh, though this is far from certain. Other races are less common, though not unknown.
Historical miscelania:
Langt-Eldheim war - some centuries ago, the Norse Kingdoms dissolved, the High Kings of Eldheimhaving become corrupt and lost the support of the Norse pantheon. The Lady Protector, who overthrew the King of Eldheim, a descendant of the dynasty that once ruled the north claims, the divine right of rulership over the surrounding area that the monarchs lost, and some of the gods support her, though not all. Many of the other local city-states were not willing to give up their independance, leading to a series of wars between Eldheim and a coalition led by Langt over the course of the last five decades.