The personal campaign setting that I use for the majority of my D&D 3.5 games is a dark weird fantasy setting, with elements of horror and grimdark; emphasis is placed on the dangerous and the bizarre. The setting draws heavily on the AD&D Planescape setting and Ravenloft settings for inspiration, along with dark fantasy books and media like Pan’s Labyrinth, the works of H.P. Lovecraft, Arthur Machen, China Mieville, and Clive Barker.
Most of the action takes place on a world called Yxra. The world itself is a continual work in progress, but here’s some basic information.
The World of Yxra
The world is slightly smaller than Earth. Yxra has two moons — a small reddish planetoid named Crax, and a larger satellite named Eredel with roughly the same coloring as Earth’s moon. Crax is closer to Yxra, with roughly a 17-day lunar cycle, while Eredel’s lunar cycle is comparable to the 28-day lunar cycle of Earth’s Moon. Crax’s smaller size causes it to have a minimal impact on Yxra’s tides — it only really has a significant impact when it lines up with Eredel and the planet’s sun.
The accepted method of timekeeping on Yxra consists of a 360-day year, divided into 12 months of 30 days each. A day on Yxra is the same length of time as on Earth. A week is 7 days, a day is 24 hours, an hour is 60 minutes, a minute is 60 seconds, and so on.
Yxra has a similar axial tilt compared to Earth, with polar caps at its north and south poles, so seasons are relatively similar. The first of the year coincides with the first day of spring, so each season, so the last day of winter is also the last day of the year.
The Grand Material
The Grand Material is the plane of existence that contains the world of Yxra, its sun and moons, the other planets in its solar system, and the full extent of the physical universe. Natural laws like gravity, time, thermodynamics, etc. work mostly the same way they do in our universe, although magic exists and does create some variations.
Alongside the Grand Material is the Warp, an infinite dimension where reality doesn’t play nice. In most ways, the Warp defies natural law and can be difficult to explain. The Warp is a vast dark nothingness. Light can exist within the Warp, but twists and roils — shadows don’t behave the way they do on Yxra, colors may appear wild and chaotic.
Time, gravity, thermodynamics, and other natural laws behave unpredictably in the Warp, or even not at all. This state may change completely at random. While living beings from Yxra can breathe in the Warp, the environment is inherently toxic and corrosive; over time, it breaks down solid matter and seemingly consumes it. There is little to no ground, terrain, or building material — what exists has been sucked into the Warp from the Material, usually in violent ways that severely damage the matter in question. What survives is usually in advanced stages of decay or corruption as the Warp feeds on it.
The Warp is, however, home to alien entities. These beings are horrific and maddening, aberrations and abominations unlike anything found in nature on Yxra. In the Warp, their forms are incorporeal and they seem to be unaffected by the Warp’s corruptive influence. Few of these entities exist as part of any particular collective race or definable group — their grotesque forms are singularly unique. They range in size from nearly microscopic to colossal monstrosities the size of planets. They do seem to have one thing in common, though: a desperate desire to escape into the Material, where they can take physical form, often with the malicious intent.
A creature of the Warp in the Grand Material exudes a corruptive, reality-warping influence on the world around it. Few Yxran creatures can endure the maddening presence of a creature of the Warp without their minds collapsing to insanity and their bodies succumbing to corruption.
Some Yxran researchers suspect the Warp itself may even be sentient, although if true, no attempts to communicate with it have been successful.
The Warp is the source of all magic, in the sense that Yxran spellcasters use a variety of methods to tap into the chaotic reality of the Warp and attempt to bend it into a desired form.
Essentially, all magic is That Which Should Not Be. A fireball spell works because the spellcaster wishes the Warp to make air and matter explode when it should not be explosive. A water walking spell works because the spellcaster wishes the Warp to make the water behave as something they can stand upon when they should sink.
Traveling into and out of the Warp is possible, but is exceedingly dangerous. Powerful mages on Yxra can create portals that lead to the Warp, but such portals are usually unstable — and there’s no telling what horrors might come slithering out. Passing through the Warp is also the only way to reach other planes of existence.