Rilutaan: The Western Marches
This is a list of terminology you are likely to hear at the table/in the game. If there are terms not listed here that you would like to have, tell me and I will get them added.
Aberrant. A type of creature that is not native to "normal" reality, either the world or any other known plane. Beholders and mind flayers are perhaps the most infamous examples.
Abyss. One of the Lower Planes, the Abyss is home to demons and consists of an infinite number of layers. The various layers are ruled over by a demon lord.
Angel. A celestial being that is functionally immortal. Angels serve at the behest of the gods of the world and are good-aligned. Angels are nearly as mythic to most mortals as are the gods themselves. It is an extreme rarity indeed for any mortal to have any kind of interaction with an angel.
Astral Plane. A sort of transition plane, it is infinite in size. The plane has several floating islands of various sizes. It is not uninhabited either, despite not being a plane native to anything (at least, not originally). It is also notable for housing portals to every other plane and, perhaps, to other worlds.
Demon. A type of fiend, demons are found in the Abyss. There are many different types of demons, running the gamut of levels of power. They are typically very chaotic in nature and incredibly violent and destructive.
Demon Lord. The most powerful kinds of demons, every demon lord is a unique, named individual who rules over one or more layers of the Abyss. Demon lords are among the most powerful beings in the multiverse. The most infamous demon lords are Demogorgon and Orcus.
Devil. A type of fiend, devils dwell in the Nine Hells. Devils vary in power and there are several different types. They are evil, but are lawful in nature, having predictable behavior. They are also renowned as deal-makers.
Devil Lord/Archdevil. A type of fiend, devil lords, or more commonly archdevils, are unique individuals of incredible power. There is an archdevil for each layer of the Nine Hells.
Domain. A domain is an aspect of existence that a god has influence over. A small amount of that influence is available to a god's clerics in the form of particular abilities and access to specific spells (Life, Light, War, etc.).
Elemental. A being made up primarily of one (or more) of the elements (air, earth, fire, or water). Elementals inhabit the Elemental Planes but can be summoned to the world.
Elemental Planes. A plane of existence inhabited by elementals, it is broken up into different regions, the Elemental Plane of Air, of Earth, of Fire, and of Water. They are often treated as different planes of existence, but are, in fact, all one plane. Where the regions intersect is a smaller region that mixes the two dominant elements. There is a point of intersection between all four major elements, known as the Elemental Chaos.
Ethereal Plane. A plane of existence that is parallel to the world such that those on the plane can still see the world, as well as people on the world. It is difficult for the planes to affect one another, though the magic missile spell is a notable exception. Travel to this plane is very easy.
Fey. A denizen of the Feywild, or a decendent of such a creature. Elves, centaurs, dryads, and pixies are all examples of fey creatures.
Feywild/Faerie. A plane of existence parallel to the world, its geography is often similar to the world, though with its natural extremes exaggerated. Cities and natural phenomena may be reflected in the plane, but they will be changed.
Fiend. A term for denizens of the Lower Planes, such as the Abyss and the Nine Hells. Typically, fiends are thought of as divided into two different camps: devils or demons. Succubi and incubi are a type of fiend as well, and fit in neither camp. There are other types of fiends as well, from other planes, such as the yugoloths of Gehenna.
Genie. A type of powerful elemental, genies come in four varieties, each corresponding with the elemental plane the genie comes from: dao (earth), djinn (air), efreeti (fire,) and the marid (water). Rumors persist among mortal races that genies possess the ability to grant wishes.
Golem. A construct that is usually (but not always) in a humanoid shape. They are constructed of different materials, giving them different properties. They are typically used for specific purposes chosen by their creators.
Immortal. This tends to refer to creatures that cannot die of old age. This is not the same as being unkillable, though immortal creatures are often quite powerful.
Lower Planes. While not literally "lower" in any sense of space with relation to the other planes, they are often given this monicker due to the planes' alignments with the evil alignments. The Abyss and the Nine Hells are the most well-known of these, though there are others. Denizens of these planes are collectively catecorized as fiends.
Nine Hells. The home of devils, it consists of nine enormous layers, each ruled over with an iron fist by an archdevil. The ninth layer's ruler is, by default, the ruler of the whole of the Nine Hells.
Plane. A realm of existence that is typically inhabited by a number of creatures (ex. The World, The Elemental Planes, the Feywild). These planes may be reachable via a portal or by other magical means.
Pocket Plane. A small, artificially-created plane of existence. They are usually small and exist outside of the world, usually made for some express purpose (such as a hideout, for example).
Portal. A sort of magical doorway used to transport people or objects across great distance or even between planes.
Underroads. A network of subterranean roads that were built by dwarves between various holds and sites. These roads and byways carve through miles and miles of caves and caverns. Many of the underroads are now lost to history and the inhabitants of the caves beneath the earth live within the Underroads as well.