As fundamentals are to elementals, incarnates are to exemplars; so goes the theory, at least. Small, possibly-living sparks of energy, these entities seem to be the pure abstract embodiment of belief unbound by physical form or mind.
The origins of incarnates likely date back as far as the planes themselves. Sightings are recorded dating back millennia, as far back as the earliest writings of mortal history beyond the Prime. Incarnates manifest in all varieties and representing all sorts of abstractions, though the most powerful are those known as the "major" incarnates, representing the core concepts of Good, Evil, Law, and Chaos, those ideas that drive the most powerful belief. Much more common are the "minor" incarnates, which include those representing such concepts as Courage, Anger, Curiosity, and Freedom; certainly significant foci for belief, if not quite as prominent. Indeed, any concept for which there exists strong belief seems to have an associated incarnate out there somewhere on the Outer Plane or Planes best corresponding to it.
Incarnates have no real interaction with the lifecycles of the planes. They spawn from raw belief, and if they feed on anything, it too is merely belief. As mentioned previously, they are drawn to strong expressions of their concepts; this may be feeding, or it may be some sort of natural attraction between similar forces. Being so strongly distinct from physicality, their mere touch is enough to wear on a person's physical health, bringing an increasing fatigue and wearing them down. When weakened enough, a being's mind is clouded enough that they can no longer fend off the presence of the incarnate, allowing them to serve as a suitable host. Those with weak enough minds are burnt out by the attempt — rare with minor incarnates, but not uncommon with major — but any with enough strength of body and of mind are empowered both physically and mentally. While their personalities are often left a little skewed by the presence (if not worse should the host's nature conflict with the incarnates's, though such a merger almost never happens) some choose to seek out such a merger despite whatever mental impact they may feel, whether for personal gain, a desire to better embody an ideal or goal, or even simple curiosity.
Much as with fundamentals, whether incarnates are living beings or nonliving constructs of belief is a common subject of philosophical debate among the biologists of the planes. It's unknown yet if they are even sapient; alone, that is. However, once merged with a sapient being, an incarnate is capable of communing with their host telepathically, speaking with them as an individual that just so happens to share their mind, aware of both their nature and their status. While this would certainly imply their sapience, the utter lack of anything greater than instinctual behavior outside of a host and their nearly petitioner-like focus on their concept brings many to think they may simply be emergent minds brought about by the combination of the incarnate's nature and the host's consciousness. As few incarnates have been specifically found inhabiting multiple hosts (merger being near-permanent barring the host's death and individual incarnates being nearly impossible to identify), and as their personalities are so broad as to be nearly identical to others of their concept, there has yet to have been any solid evidence in either direction.
As mentioned, incarnates merge for life, a constant mental companion for the rest of their days unless expelled by certain specific forms of magic. Once merged, the incarnate urges their host to express their concept through behavior and action even more than they likely already have, pushing them towards it even to the exclusion of all else. If need be, they can even directly control their host, though as incarnates tend to merge with those that already express well their concepts such extremes are rarely necessary. Still, this often serves as a surprise for psions or mentalists thinking they've put out a host's consciousness only to have the incarnate suddenly take hold.
Outside a host, an incarnate is utterly invisible to normal sight, the only sign of their presence being the dull fatigue associated with their touch. Even with magic, they appear as little more than a ball of flickering light an inch or two across, flitting about like a moth and shifting through various colors at random. Once within a host, they rarely show external sign of their presence, though on occasion when the incarnate expresses its abilities or takes hold of its host, a dim glow will be noticeable in the host's eyes.
- Planescape Monstrous Compendium, pp.58-60