The underlying nature of reality in Sea of Shadows is very different from the real world. The universe is effectively flat (there are no planets, no "space", etc.), with the Sea of Shadows connecting all the realms together. Each realm can have very different characteristics than the others. The nature of the sun, moon, and stars can vary from realm to realm (they are usually powerful spirits or elementals high up in the sky, and some realms may not even have a sun, moon, etc.). The nature of seasons and weather can vary wildly as well (many realms have no seasons and can even have persistent weather that barely varies at all). Light and matter interact through magical means, and even the biology of living things is very different when you look beneath the surface. The study of this underlying reality is called “alchemy” and includes everything that would be considered a science in the modern world.
Time flows at the same rate throughout the universe (there is no time dilation at high speeds, or other relativistic effects). Time cannot be manipulated, even by magical means (no time travel, etc.). Precognition and postcognition are only possible in the sense that an educated guess can be made about the future or past (and with magic, this guess can be very educated). Normal aging is not inevitable. While most people age and die normally, heroes can often live for centuries or even longer, aging at only a fraction of the normal rate (through a combination of force of will, longevity potions, restoration spells, etc.). The age of the realms varies, but many are at least tens of thousands of years old.
THE CALENDAR: While individual realms might have local calendars of their own, most travelers and educated people use the alchemist's calendar. In this calendar, there are 13 months each year, each consisting of 28 days (each named after an animal: boar, stag, ox, horse, hare, turtle, serpent, crane, hawk, owl, panther, wolf, and bear). There is also a single day at the beginning of each year (not part of a month) that is considered a holy day and is dedicated to religious rituals and feasts. Some realms have seasons, but many don't, or have seasons that are based on the weather, not on the temperature. Whatever overall cycles a realm has, though, they all revolve around a 365 day year and a 24 hour day (even if the realm does not have a normal day-night cycle). Years are designated from the official founding of the City of Haven, 1820 years ago.
Mana is the fundamental force that animates everything, and is divided into five resonances: arcane mana, primal mana, nature mana, holy mana, and infernal mana. A quantum of mana is called a "mote" and acts as a transmission "particle" for all forces, although they act instantaneously and have no mass. Souls are made up of mana that is usually tied to a general physical entity (i.e., a body), though they do not have to be. A soul by itself cannot think and has no memory or will.
In general, infernal mana is destructive, holy mana is restorative, nature mana is transformative, primal mana controls earth and fire, and arcane mana controls water and air. Using mana in a powerful and overt way is called magic, but all life uses mana to live, move, and think (they just do not do it consciously and with training).
Matter is made up of the four fundamental elements: earth, fire, water, and air. A quantum of matter is called an "atom" and cannot be divided further (there are no electrons, neutrons, etc.). Atoms are technically point particles, but in practice they have a “size” because they cannot get too close to one another. Earth atoms have a mass of four, relative to a water atom's mass of one. Air atoms have a mass of zero (making them weightless), while fire atoms have a mass of negative one (making them even more insubstantial than air). All physical objects (living or not) are made up of varying amounts and combinations of atoms. While matter often acts in a manner similar to the real world, it (and the forces of mana) can end up acting very differently.
GRAVITY: All matter with positive density is pulled downward by the primal force of gravity. Gravity has an acceleration rate of 5 meters per second squared (in the absence of any other effects, such as air resistance), which means it is considerably less than the normal world (so things can be thrown farther, longer jumps can be made, falls are less dangerous, etc.). Matter with negative density is pulled upwards instead.
LIGHT: Light is a pseudo-particle made up of an atom of fire paired with a mote of mana called a pulse. This is somewhat similar to a photon, but the color of light has nothing to do with wavelengths (quantum mechanical wave-forms do not exist). An arcane pulse is blue, a primal pulse is red, a nature pulse is green, a holy pulse is white (enhancing other pulses), and an infernal pulse is black (dampening other pulses).
Rational thought is performed with pulses of light, although this usually requires a physical brain in which to take place (a spirit, such as a ghost, would be a soul and a mind without a body). Different types of thought tend to use different light pulses, as appropriate to the resonance of mana involved.
Pulses of light have infinite velocity (which means the laws of physics are basically Newtonian, not Einsteinian). This means that in some sense they do not even exist, except that the mote of mana can affect what it passes through, and the fire atom can end up being embedded in another object (heating it up, while cooling down the object it was removed from). Heat works through this mechanism, as fire atoms (being almost completely insubstantial) can diffuse themselves throughout things made of other types of atoms.
The four fundamental matter atoms are the four primary elemental substances when they are not combined with each other. Combinations of two different types of matter atoms are considered “secondary elements”, while combinations of three matter atoms (with at least two of the three atoms being different types) are considered “tertiary elements”. The only stable combinations of four or more matter atoms are metals (one atom of each type, plus two of any type) and living tissues (chains of particular patterns of matter atoms).
AIR: What the atmosphere is made of. Pure air is a light blue color, but you have to have miles of it to see this. Note that air pressure is always one atmosphere (at any altitude), since air has no mass. Air has a density of 0.
WATER: What the rivers, lakes, and sea are made of. Pure water is also a light blue color, but you have to have several meters of it to see this. Water has a density of 1, a freezing point of 30 degrees, and a boiling point of 200 degrees.
EARTH: Pure earth is known as obsidian. Pure obsidian is quite hard, has no air pockets, and is a deep black color. Earth has a density of 4 and a melting point of 500 degrees.
FIRE: Pure fire is unstable and emits pure white light pulses while being pulled upwards. Pure fire can only exist temporarily as hot plasma. Fire has a density of –1.
CLAY (EARTH+WATER): Pure clay is a soft, almost fluid, brown substance. Heating it removes water and makes it more like rock. Clay has a density of 2½ and melts at 250 degrees.
DIAMOND (AIR+EARTH): The purest gem, extremely rare and valuable. Diamond has a density of 2.
ETHER (AIR+WATER): A heavier than air mist/fog. It suppresses fire, though it can be overcome if the fire is intense enough. It also knocks out anyone who breathes it (in sufficient quantity), but has no other ill effects. Ether has a density of ½.
FIREWATER (FIRE+WATER): A highly flammable, clear liquid, similar to oil. It is as light as air, so if sprayed as a mist you’ve got a fuel-air explosive. It is usually mixed with regular water so that it is easier to handle. Firewater has a density of 0 and ignites at 150 degrees.
INFERNUM (AIR+FIRE): A lighter than air gas, very flammable and unstable (even shocks could set it off). It slowly breaks down into air and fire due to slight vibrations, giving off a very slight glow. Infernum has a density of –½ and ignites at 100 degrees.
LUMENUS (EARTH+FIRE): A soft rock that emits a slight glow. If ignited, it burns furiously until it is completely consumed. Lumenus has a density of 1½ and ignites at 200 degrees.
NIMBUS (FIRE+FIRE+WATER): When two light pulses (which include fire atoms) simultaneously hit water, they often combine with it to form a "water vapor" called nimbus. This vapor slowly rises and joins with other nimbus particles to form clouds. When too much accumulates, it starts to rain. The fire atoms released in this process join with air atoms, causing lightning. Nimbus has a density of -⅓.
ALCOHOL (WATER+WATER+FIRE): Normal alcohol, but is harder to light than “real” alcohol (it also weighs a lot less). Alcohol has a density of ⅓ and ignites at 200 degrees.
CERAMIC (EARTH+EARTH+WATER): Normal ceramic, but it is usually made with air pockets to make it lighter. Ceramic has a density of 3 and a melting point of 400 degrees.
CHLORINE (AIR+AIR+FIRE): A very corrosive, lighter than air gas that is completely transparent. Chlorine has a density of -⅓.
ACID (AIR+FIRE+WATER): Pure acid will dissolve an equal amount of anything (barring magical help). Impure mixes of acid dissolve less and can be contained. Acid has a density of 0.
COAL (EARTH+EARTH+FIRE): Normal coal; density of 2⅓ and ignition point of 300 degrees.
FOG (AIR+AIR+WATER): Normal fog; density of ⅓.
GLASS (AIR+AIR+EARTH): Glass is transparent, light and brittle. Glass has a density of 1⅓ and a melting point of 300 degrees.
IRONWOOD (EARTH+FIRE+WATER): Ironwood is not actually wood, but is a very rare wood-like substance found underground near very old trees. It is very strong, sinks in water, and has a jet-black color. Ironwood has a density of 1⅓ and ignites at 400 degrees.
LIGHTNING (FIRE+FIRE+AIR): Lightning is not electricity, although it has some similar properties. However, it cannot conduct through solids or liquids; real electricity does not exist in world of the Sea of Shadows. Lightning has a density of -⅔.
LUBRICUS (WATER+WATER+AIR): A nearly frictionless liquid, lighter than water and non-flammable. Lubricus has a density of ⅔, a freezing point of 0 degrees, and a boiling point of 150 degrees.
PYRITE (FIRE+FIRE+EARTH): A light rock-like substance that burns constantly (without the need for lighting), unless it is completely encased in a non-flammable substance. It is commonly used for lighting or torches, and leaves behind only earth as it burns (no air is consumed in the burning process). Pyrite has a density of ⅔.
QUARTZ (EARTH+EARTH+AIR): Normal quartz; density of 2⅔ and melting point of 750 degrees.
RUBY (AIR+EARTH+FIRE): Rubies are used as ammo for firearms. Striking a perfect ruby at the exact right angle and spot will drive out all of its fire atoms in a brilliant flash. The remaining unstable diamond (Air+Earth) is launched forward at the speed of sound, exploding into rock fragments when it hits. Ruby has a density of 1.
EMERALD (AIR+EARTH+WATER): Normal emerald; density of 1⅔.
BLOOD (WATER+WATER+EARTH): Normal blood; density of 2, freezing point of -50 degrees, and boiling point of 250 degrees.
Adamant (made of one of each type of atom) is the pure, primal metal that forms the basis for all other metals. Metals consist of an adamant atom with a fire, earth, water, or air atom on the top and on the bottom. For purposes of mass, the atoms that make up adamant normally cancel each other out, but when two additional atoms are added, this balance is thrown off and amplified, making metals the heaviest of the elements. All metals (but especially adamant) are very difficult to manipulate with magic.
ADAMANT (EARTH+WATER+AIR+FIRE): Effectively weightless and incredibly strong, pure adamant is completely reflective, having the appearance of a perfect mirror. It has no temperature (it feels like skin temperature) and cannot melt. It is immune to magic and is almost impossible to work with or create (it never occurs naturally). Adamant has a density of 0.
COPPER (ADAMANT+WATER+EARTH): Moderately rare, used in coins (worth about a dollar) by the lower classes. Copper has a density of 8 and a melting point of 1000 degrees.
SILVER (ADAMANT+WATER+AIR): Fairly rare, used in the coins (worth about 10 dollars) favored by merchants, knights, etc. Silver has a density of 12 and a melting point of 750 degrees.
GOLD (ADAMANT+WATER+FIRE): Quite rare, used in the coins (worth about 100 dollars) favored by the nobility. Gold has a density of 16 and a melting point of 500 degrees.
QUICKSILVER (ADAMANT+WATER+WATER): Very similar to mercury and extremely rare. It cannot be made into a pure coin (it would melt), but it is made into a 50% gold/quicksilver alloy called Electrum (worth about 1000 dollars per coin). Quicksilver has a density of 20 and a melting point of 0 degrees.
LEAD (ADAMANT+EARTH+EARTH): The most common earth metal. Lead has a density of 24 and a melting point of 1500 degrees.
IRON (ADAMANT+EARTH+AIR): Normal iron. Iron has a density of 8 and a melting point of 2000 degrees.
STEEL (ADAMANT+EARTH+FIRE): Steel is not an alloy, but is made by melting iron down so that iron’s air atom is replaced by a fire atom. Steel is the strongest, hardest metal next to mithral and adamant, but it can “rust”. If left in open air, its surface will slowly turn back into iron. If left in water, it will slowly turn into copper. Steel has a density of 6 and a melting point of 2500 degrees.
IRONGLASS (ADAMANT+AIR+AIR): A transparent metal with slightly less clarity than glass. It is fairly strong, but brittle. Ironglass has a density of 4 and a melting point of 750 degrees.
CHROME (ADAMANT+AIR+FIRE): A very shiny metal that binds instantly to steel when melted. Most steel weapons and armor are given a layer of chrome to prevent rust. Chrome has a density of 3 and a melting point of 500 degrees.
MITHRIL (ADAMANT+FIRE+FIRE): The lightest normal metal, also extremely strong (stronger than steel, but weaker than adamant). It never melts and is easy to enchant. Metal magic weapons/armor are made of mithril. Mithril has a density of 2 and has no melting point.
There are four basic types of living tissues, each of which consists of a string of adamant atoms linked together by earth, water, air, or fire atoms (in a manner similar to metals). These “elements” can only exist as chains. You can’t have a single muscle or bone particle, because it would instead be a metal particle.
Different patterns of connections (such as earth, fire, earth, fire, or water, water, air, water, water, air) result in the nearly infinite variety of living tissues beyond the basic tissues listed below. Note that there are no cells, microbes, viruses, etc. Living things are animated by mana, not by the mechanisms of normal biology. Living tissue is extremely difficult to manipulate magically due to its complexity (even more so than metal).
BONE (ADAMANT+EARTH): Bone is the structural basis for animals. Bone has a density of 1.4 and a melting point of 350 degrees.
MUSCLE (ADAMANT+WATER): Muscle allows a living being to move. Muscle has a density of 1.1 and a melting point of 300 degrees.
NERVE (ADAMANT+AIR): Nerve (and brain) tissue transmits light pulses. Nerve has a density of 1 and a melting point of 250 degrees.
WOOD (ADAMANT+FIRE): Wood is the structural basis for plants. Wood has a density of 0.9 and ignites at 200 degrees.