Messengers

The Messengers are the native denizens of the Borderlands, and as such they occupy a unique position, halfway between the chaos of the Changing Ones and the stability of mortal species. Like the Changing Ones, they are capable of changing shape with little to no effort; like mortals, they are also capable of retaining a shape without effort, and of having a stable and recognizable personality. In addition, the Messengers are capable of bringing some elements of one realm into another. When they bring some of the mutability of the Mutable World into the Bounded World, mortals generally call the ability "magic," though it is perfectly natural to Messengers. (What, if anything, the Changing Ones call it when they bring temporary stability to the Mutable World is unknown.)

Physiology and Appearance

Due to their nature, Messengers are capable of assuming various shapes, and can change shape readily. This ability is nearly effortless and nearly limitless in the Borderlands. However, by its very nature, the Bounded World resists shapes that defy its natural laws. It is therefore very easy for a Messenger to assume a shape that would 'work' according to natural laws, and in many cases Messengers take the appearance of natural animals or plants, including humanoids. Changes that do not defy natural laws are not uncommon--it is just as easy for a Messenger to appear as a blue horse as it is to appear as a brown horse, for instance, or to appear as a human with horns as to appear as a human without horns. Even a very young, very inexperienced Messenger can assume the shape of, for example, a tiger or an oak tree and maintain it for a very long time. 
Changes that defy natural laws are more difficult. The dragons of northern Eale would be incapable of flight and fire-breathing if they were natural, mortal creatures. Assuming the shape of a dragon briefly is not difficult for a Messenger, but assuming the shape and maintaining it over more than a few minutes is very difficult indeed. Thus, most dragons are older and more powerful Messengers.
Bodily form has some impact on the minds of Messengers as well, and the longer they spend in the shape, the more pronounced the effect. A Messenger who frequently assumes the shape of a trickster imp will gradually acquire more and more traits and mental habits of a trickster; a Messenger who frequently assumes the shape of an angelic creature of healing will gradually develop more traits of compassion and nurturing. Messengers often select their shapes with these facts in mind.
One other factor has a major impact on the Messenger's shape: human beliefs in the area. Belief is a potent factor that can counterbalance the inertia effect that causes Messengers to readily lose more 'impossible' forms. The clearer, stronger, and more consistent the belief, and the more people in a region hold that belief, the more pronounced the effect. For this reason, almost all Messengers take a shape that is at least not contradictory to local belief, and generally take a shape that is actively congruent with it. As assuming and maintaining a form becomes easier the longer that form is kept, most Messengers confine their interactions with the Bounded World to specific areas.
Examples: 

• In northern Eale, belief in dragons has resulted in Messengers taking draconic forms when appearing there.
 
• The people of Enzia believe in the Aia, who are powerful and potentially terrifying beings of healing and justice. Messengers who manifest in those areas have an easier time with many kinds of magic than do Messengers in other regions, and are physically invulnerable, but are also subject to the extremely strict code of behavior believed of the Aia.
• The Eshthali believe in animal spirits called Beast-Angels that often bond with specific humans to enable them to perform magic. Messengers who manifest on the Eshthal Islands generally take an animal form.
• The Forest of the Unknowable Ones is believed by locals to host capricious, whimsical creatures similar to faeries or elves, who may have a humanoid form, an animal form, or both; Messengers take forms (and, with them, personalities) conforming to these beliefs about the Unknowable Ones.

Magic and the Messengers

Magic enters the Bounded World exclusively by the agency of Messengers, though in some areas Messengers will bestow magic on humans that can continue to work outside their immediate presence. Magic is essentially the bringing of some element of the Mutable World to the Bounded World, and thus permitting humans to make changes that would otherwise be impossible. Just as a Messenger's shape is in part controlled and affected by the beliefs in the area, the way that magic works is similarly affected. For example, in the Eshthali Lands, magic is believed to be possible by the direct intervention of Messengers, or by the writing of magical glyphs by humans; thus, Messengers can directly perform magical acts, or can power the glyphs written by humans. Though powering these glyphs requires effort of them, they are incentivized to do it because if magic stops working, belief in the animal spirits may fade, making it more difficult for them to effectively act in the Bounded World.
See Magic for more information.

Relationships with Mortals

The degree to which Messengers relate to mortals depends in part of their personal proclivities and in part on the belief system of the part of the world in which they operate--and the former often informs the latter. A Messenger who is interested in a direct, personal relationship with a mortal might chose to manifest in Eshthal, where belief in Messengers bonding with humans is strong; a Messenger with no interest in that kind of close relationship might manifest in the Forest of the Unknowable Ones, where the relationship is much more indirect. 
Some Messengers are not interested in humans at all, and choose to manifest in areas with little or no human presence--for instance, at the ice cap, or the bottom of the ocean. These Messengers have neither the benefits nor the detriments of manifesting in proximity to humans: they are not constrained by local belief, but they also don't have the 'boost' to their magic that local belief provides. Other Messengers are perfectly content to remain in the Borderlands and not interact with the Bounded World at all.

Behavior and Life Cycle

Messengers live for thousands of years, and functionally cannot be killed, though they can be weakened sufficiently that they are incapable of leaving the Borderlands for months, years, or even centuries. Existing in the Borderlands is effortless to them (though some of their other abilities may become impossible for them if they are sufficiently depleted of energy), but existing even for a few minutes in the Bounded World or the Mutable World requires effort, and if their power is drained, they will be unable to expend that energy. A Messenger's potency can be depleted by manifestation in either of the other realms (especially in a particularly incongruous form), by use of magic, and by reproduction. It will gradually 'grow back' of its own accord in the Borderlands; simple existence in the Borderlands is enough to effect this, though Messengers can hasten the process by consuming the material of the Borderlands. 
The simplest way to force a Messenger back to a harmless state in the Borderlands is to force them to expend a significant amount of magic; this works in any of the three realms. Whether a Messenger can be physically harmed in the Bounded World depends on the rule that local belief put on its form. For example, the dragons of northern Eale are believed to be vulnerable only to certain very specific attacks (generally, stealing their heart or carbuncle will render them vulnerable to physical harm the Unknowable Ones are believed to be vulnerable to being bound via trickery, but not to physical harm; and the Aia of Enzia are believed to be wholly invulnerable, though they are bound by other rules that can be at least as restrictive. Those whose local belief systems renders them physically invulnerable can still be forced back to the Borderlands.
Messengers who have taken forms that are expected to eat mortal food do eat it. Beast-Angels are given physical offerings of whatever food would be a choice meal for the animal shape they took, and the Unknowable Ones eat many things, but are particularly fond of bread, milk, and flowers. Messengers who have taken forms that are not expected to eat generally do not eat, but they can do so for pleasure, or for a particular purpose; the Aia eat only to respect hospitality. These foods have no physical effect, but may help the Messenger conform to local belief. The actual sustenance of a Messenger is the material of the Borderlands, which they can absorb gradually through exposure or more quickly by consuming some of the changeable matter there.
Messengers reproduce only in the Borderlands, and not bodily. Instead, two or more Messengers donate a small portion of their potency to a new being. Newborn Messengers are fully physically mature, but must be taught the rules of the three worlds and the ways of using magic; this can take quite some time, and requires all of the parents to assist. Messengers do not reproduce frequently, as this task is lengthy and requires a great deal of energy.
Messengers do die, after a certain number of years have passed. The exact number is fixed and unknown, though it is very clearly well into the thousands. Messengers are not inherently more intelligent than mortals, but this extremely long lifespan means that they have more time to practice various skills, and more experience to draw from, giving many of them the appearance of greater intelligence.

Messengers and Religion

Messengers are not infrequently mistaken for divine or semi-divine beings, especially those who present as powerful and invulnerable. Many Messenger schools of philosophy do not encourage attempting to reinforce this belief, though often (as with the angelic Aia) this is difficult. On top of this, most Messengers find 'playing god' to  be appallingly egocentric or just plain crass. This has not prevented some Messengers from setting up cults devoted to their own worship, though they are always at risk of other Messengers arriving to pop that bubble--if only out of personal offense at the Messenger's ego.
Messengers have many schools of philosophy and religions, and some worship gods of their own; those who have spent a great deal of time with mortals may also convert to mortal religions. Some Aia, for example, have converted to the worship of the Unseen God. As with mortal religions, the truth or falsehood of any given Messenger religion is unknown.
Just as some mortals worship Messengers, some--both mortals and Messengers--worship the Changing Ones. The cult of the Changing Ones is small and generally considered to be misguided at best, but is fervent in its beliefs. Though most worshippers of the Changing Ones are peaceful and harmless, they are viewed with wariness, as in the past some devotees have attempted to open permanent portals to the Mutable World, with disastrous results. The Changing Ones appear to be wholly indifferent to worship, just as they seem to be wholly indifferent to most behaviors and beliefs outside their realm.