This one isn't so stream-of-consciousness, since I had some time to think about it before starting. Oops. I try to not do that, but sometimes it happens anyway.
There are things that have been destroyed and written out of reality: histories that no longer happened, colors that don't exist anymore, emotions that nobody can experience now, lifeforms that never walked the Earth... Perhaps they're what was erased from the drawing board once God decided what would be included, or the remnants of dead universes, but they existed once and now they have ceased to exist so completely that for all intents and purposes they never existed.
These concepts integrate themselves into people and other things, sustaining their half-existence on a full, complete one, and existing in a symbiotic-bordering-on-parasitic relationship (they rarely care about helping beyond whatever is necessary to keep themselves "alive", but if you can adjust to how you have been changed then you are-- mostly-- in control). It is an odd existence, being the host for an alternate history, an element that used to exist on the periodic table in some never-was universe, or a two-dimensional lifeform, and it warps the way that you think and the way that you interact with the universe.
It puts you in danger, too, because there's Something Out There that doesn't like all of these loose ends running around and is trying to clean them up. Not to mention that sometimes your parasite-concept gets the better of you and directs you in ways that you don't understand (sometimes don't even recognize). It's one thing to be the host for a nonexistent color and quite another one to be the host for a nonexistent color that sometimes needs people to just... die.
Parasite-concepts have some control, but not much. They don't have much chance to float around on their own so after losing a host they are limited mostly to whatever is in the immediate area, unless they want to take some big risks. They are also somewhat limited by their hosts: their thinking power isn't much better than the thinking power of the host, so inhabiting a human gives them an edge in surviving. A pyramid-bound parasite-concept is for all intents and purposes merely existing-- it doesn't even have enough self-awareness to know that it exists.
How did they gain sentience, or even enough instinct to know that survival is important? I guess we'll have to chalk that one up to them being... Well, they're Something, and "magic" is just as good of a descriptor as any other, but the most accurate statement would probably be to say that they're inherently weird and just leave it at that.
There are things called Symphonies, which are groups of multiple parasite-concepts that are inhabiting the same host for some reason. Either they are linked in some way (a song and the nonexistent emotion that it was written to inspire) or they find that they complement each other in some fashion and/or have similar goals (far less common, and usually much more temporary). They probably aren't common at all. If this concept were for a trilogy then there would be only one, maybe two, of them in the whole series, and definitely not among the main characters.
Symphonies are also not known for their mental stability.
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