Cosmology

The world of the Messengerverse has three distinct strata, though the way they are categorized and described vary from country to country and culture to culture. In its most basic form, the Messengerverse is divided thus:

• The Bounded World, often called simply 'this world' or 'the real world.' The Bounded World is the world of physical matter and predictable natural laws, and the home to mortal species. Though it has many names in many languages, often emphasizing different parts of its nature (the Talyans call it the World of Necessity because it is a place where reactions necessarily rather than randomly follow from actions; the Ealians call it the Knowable World, because it is capable of being comprehended with time and effort; the Te Moi call it the Moral Lands, because they perceive morality as being only possible in a realm of predictable consequences), the most common term used by scholars and metaphysicists is some variation on the Bounded World, because it is the world constrained in space, time, and cause-effect relationships. (Belief in the stability and solidity of the Bounded World does not preclude belief in a spirit or soul existing for mortals, but that spirit or soul is considered to be subject to rules just as physical matter is.)
• The Mutable World, home of the Changing Ones. The Mutable World has little concrete physical matter and no predictable natural laws; it is a place entirely given over to constant change. It, too, has many names in many languages--the Capricious World, the Changing Lands, and even, sometimes, Hell--because contact with the Mutable World is disastrous to creatures of flesh, warping and twisting them when not killing them outright. (As a corollary, the Changing Ones are equally unable to survive in the Bounded World. Their changeable, suggestible flesh is torn asunder by the pitiless laws of the material world.)
• The Borderlands, home of the Messengers. The Borderlands are a middle ground between the predictable materiality of the Bounded World and the chaotic change of the Mutable World. Its inhabitants, the Messengers, balance between those two extremes, able to exist in either world, but not fully at home anywhere but the Borderlands in between. Some metaphysicists believe that the Borderlands was deliberately created by the gods to protect the Bounded World from the Mutable and vice versa; others believe that it was an accident, the result of the two disparate sets of physical reality leaking into each other and creating a stable middle ground. Because it partakes of both the predictability of the Bounded World's rules, and the power of the Mutable World's changeability, the Borderlands are the realm of magic, and the Messengers, the agents of magic.

Many religions in the Bounded World also posit additional worlds in the form of realms of the Gods, afterlifes, elemental or infernal planes, and so on, but unlike the three strata above, their existence has never been materially proven.
Travel between the realms is difficult too impossible except for the Messengers, who are native to the Borderlands but capable of surviving in either of the other worlds as well. For Messengers, travel between the worlds requires a minor effort, and travel to an area in which they previously spent time is easier than travel to an area in which they infrequently or never spent time. Messengers can stay in the Bounded World or the Mutable World for varying lengths of time depending on their familiarity with the area as well as their relative age and power--younger Messengers may be limited to months, whereas older Messengers may be able to stay for hundreds or even thousands of years.
Mortals are capable of visiting the Borderlands via certain portals or with the aid of the Messengers, but only for limited periods of time lest they develop border sickness. The Changing Ones have a similar ability, though it is unknown exactly what restricts them to a time limit. Mortals cannot survive for more than a few seconds in the Mutable World; at best, the raw chaos will warp and kill them outright, at worst, transform them. Similarly, Changing Ones cannot survive more than a few seconds in the Bounded World, as its sheer physical concreteness shreds their ephemeral bodies. Due to these constraints, mortal cultures may label the Mutable World as a hell or hell-like dominion based on brief exposure; what the Changing Ones think of the Bounded World is unknown.

Manifestation

The Bounded World is the "real world" of natural laws and ecosystems. It is roughly earthlike, with similar biomes, geographical regions, seasons, and ecosystems--ocean, forest, desert, taiga, tundra, glaciers, grasslands, and so on--and animals and plants similar to those on Earth. Due to the influence of the Messengers, the Bounded World does have magic, but a kind of metaphysical inertia makes magic degrade over time, reverting to natural laws unless deliberately maintained either by mortals or Messengers.
The Mutable World presents itself to humans as a swirling chaos of color and form, with objects shifting in and out of existence as the seconds pass, and time flowing more quickly or more slowly at random. Messengers perceive it in a similar way, but just as they are capable of bringing the mutability of magic to the Bounded World, they can bring stability to the Mutable World, causing regions to become more concrete and predictable for limited periods of time. How the Changing Ones perceive it is unknown, though they do not seem to find it unpleasant in the way that mortals do, and seem to have the ability to direct (though not stop) the constant flux. Whether the Mutable World has living beings besides the Mutable Ones (or, in fact, whether the whole realm is itself somehow alive) is also unknown.
The Borderlands presents itself as a hybrid of the two: regions of stability that nonetheless may change rapidly due either to random chance or to the whims of the Messengers who live near it. For example, a region of the Borderlands might appear to be a desert plateau one day, and then a forest glade the next--and the regions have no necessary connection to one another: one might travel directly from a hot desert plateau to a freezing glacier over the course of a few steps, and space and time are not completely predictable. Absent Messengers, these changes are gradual but whimsical. Messengers can direct and control the changes, but must regularly visit both the Mutable and Bounded Worlds to learn and maintain the ability to do so; essentially, Messengers borrow change from the Mutable World, and stability from the Bounded World, and use both to manage their environments.

Localization

Though the thee realms are normally entirely distinct from each other, with only the Messengers traveling between them, powerful magic has opened certain portals between the Bounded World and the Borderlands. In these areas, it is possible for humans to travel to the Borderlands, and there is often an area of magical 'leakage' around the portals that renders a certain section of the Bounded World a little less predictable by way of natural laws. Keeping these portals open permanently, rather than temporarily for a specific purpose, is considered a faux pas by most Messengers, but it does happen. Portals can also be opened between the Mutable World and the Borderlands, but due to the very nature of the Mutable World, they tend to close themselves sooner rather than later; keeping them open requires effort on the part of the Messenger or Changing One who opened it.
There are no consistently-open portals between the Mutable World and the Bounded World.