Every night, you lose around 80% of your memories from the day before; they end up discarded, thrown away, deemed useless.
Your memories resent that. They're angry about that.
And, in their anger, they hunt.
The Nightly Harrowing.
Basically, whenever somebody goes into REM sleep for the first time in a night, the memories they're throwing away form into a cohesive sphere of mnemonic-matter (originally shortened to mne-ma, a phrase eventually warped by common usage to mana). It darts out from your head, and you find yourself possessed by it. It controls your every move. You share it's 360 degree vision (with mana looking solid and colorful and the real world being translucently and monochromely overlaid) and can give it telepathic advice and chit-chat on ideal feeding strategies. It can move at precisely 5 miles per hour. It wants to feed.
People and animals emanate mana throughout their day to day existences, and that mana generally condenses into little pellets hanging in the air. Your Little Mana Sphere (hereafter and colloquially referred to as a Llama, because language is weird) can gorge and grow quite a bit by eating (or absorbing, really) those pellets, but after a while it's big enough that the pellets are a drop in the bucket instead of a full meal, and it... wants more.
Your llama... wants to eat other llamas! (And possibly you, but that goal is less attainable).
Basically, it goes out into your surrounding environs and try to intersect with other, smaller llamas in order to absorb them, as a sort of three dimensional game of tag, growing in the process. If your llama gets eaten, you go into normal sleep and can dream normally, with the memories that form the llama gone.
But wait! What if it manages to last the full night without getting eaten? In that case, your current llama goes up in a puff of metaphorical smoke, appears besides you, and reenters your head, un-discarding the memories it was formed of and searing them into your brain with eidetic clarity. Your llama also stops being semi-maliciously sentient at that point. The mana that your llama accumulated (and that initially composed it) attaches itself to the involved memories, and you can use it to perform... dramatic pause... magic!
Doing magic.
The mental interface for accessing your mana looks like rows and rows of spheres, one for each llama you've absorbed. They have little animations of the memories they're attached to playing on them, and their size is proportional to the amount of mana originally contained in them. You mentally 'poke' a sphere to do magic with it; it's considered polite to announce what you're casting (and make some hand motions) while you cast it so you don't surprise anyone.
Casting a spell is approximately instant, but your brain recoils from it enough that most people take around six seconds till their brain is clear enough to cast again, with an approximate standard deviation of two seconds (people with no recoil time are very rare and are normally strong masochists). The recoil is the product of your brain getting a strong, temporary impression of the unimaginable pain a Llama goes through while being eaten by your brain. Llamas have the same basic pain reactions as their creators, so people with higher pain tolerances or more favorable reactions to pain don't get blasted as much. People who are completely non-masochistic with normal pain tolerances occasionally also lack recoil or have exceptionally low recoil, but much more rarely (which is saying something).
The spheres gradually lose... themselves, as mana is drained from them (imagine a tinted glass ball with water draining from it. Once it's completely drained, a sphere 'pops'. Memories attached to spheres become less eidetic as you drain mana from them; at 87% they're around normal 'happened yesterday' clarity, at 75% they're 'happened a month ago clarity', at 50% they're 'happened a year ago' clarity, at 25% and below they're 'I have a blank spot in my memory, I can't even remember thinking about it, oh gods the discontinuity! What if I had unprotected sex!'. Once a sphere is completely drained it pops like a soap bubble.
Magical specialties.
There are nine essential elements (and their associated emotions); Positive (Pride), Mind (Disgust), Nature (Sadness), Water (Surprise), Air (Fear), Fire (Trust), Earth (Joy), Metal (Anticipation), and Negative (Anger). Any given person has a magical purview, determined by the primary emotion they're generally most prone to, then they have a superspecialty, determined by the primary emotion they're second most inclined to (see the table/chart/whatever), and then they have a specialty, determined by the emotion they're third most inclined to, and a subspecialty, determined by the emotion they're fourth most inclined to.
You halve the mana cost of spells within your purview, quarter the mana cost of spells in your superspecialty, 1/8th the cost of spells within your specialty, and 1/16th the cost of spells within your subspecialty.
(I do not currently have the wherewithal to create 273 subspecialties and 819 sub-subspecialties, so those are sort of... less defined.).
The reason that people have specialties is that your general emotional patterns and personality 'color' the sorts of llamas you create.
Example: Someone (cough cough) could have purview over Positive energy, then have a specialty in Healing magic, then have a subspecialty in Fertility magic, then have a sub-subspecialty in Gender-Healing magic, which would be used for gender-shifting spells that bring someone closer to their current ideal form.
Specialties
Summaries for specialties:
Positive magic involves summoning and binding celestial beings (which lets you create magic items), creating (voluntarily or involuntarily) binding contracts, healing magic, and magic empowering physical attributes.
Mind magic involves manipulating and reading emotions and thoughts, illusions of various sorts (tactile illusions being lumped under hearing and illusory pleasure being lumped under pain), metamagic, and telepathy.
Nature magic involves control over basically every life form that isn't sapient; you can summon, kill, create, increase the fertility of, make grow really fast, et cetera everything from microbes to koalas. Shapeshifting into animals and plants included.
Water magic involves magical shielding (mental or magical), hydrokinesis and the creation of cold, the reduction or expansion of distance, depth, or size (AKA space magic), weather magic, and the control of oozes.
Air magic involves control and summoning of the 'neutral' outsiders (spirits, inevitables, and proteans), the creation of genuine sound and vibration, and the manipulation, creation, and destruction of gravity and air in general.
Fire magic involves pyrokinesis (both in the creation of flame and the creation of heat), making subjective time go faster or slower, precognition and postcognition, and turning life into an action movie.
Earth magic involves the control, creation, improvement, destruction, and et cetera of various forms of mineral, alongside turning into minerals (this is always an option, but it's particularly easy when earth is involved) and lava. LAVA. and salt. SALT.
Metal magic involves electrokinesis, control of both precious and normal metals, technokinesis, acid and alkalinity manipulation, light creation, and the creation, manipulation, and destruction of oils and plastics.
Negative magic involves summoning and binding infernal beings, creating and controlling the undead, the destruction of light and the animation of shadow, the dispellation of other magic, and the weakening and harming of someone's physical attributes.
Miscellaneous.
The average person produces about 2,000 mana pellets per day of activity, and the average Llama starts out ‘weighing’ 100 mana pellets (originally shortened to Ma.Ps, someone clever called them cartographers, that changed to cart of gophers, now they’re just called Gophers. Thus; mana is measured in gophers.). Speaking in DnD terms, a cantrip would cost 8 gophers, a level 1 spell would cost 64 gophers, a level 2 spell would cost 512 gophers, a level 3 spell would cost 4,096 gophers, a level 4 spell would cost 32,768 gophers, a level 5 spell would cost 262,144 gophers, a level 7 spell would cost 2,097,152 gophers, and a level 9 spell would cost about 32,777,000 gophers (note that cost goes up disproportionately compared to power).
That's just a sort of generic way of approximating spell power, rather than a concrete rule; the actual rule is that it is exponentially harder to use mnemonic-matter to influence actual matter the more matter you're influencing at once.
Most people wear full BDSM bondage gear (gag and straps included) to bed, in order to keep their llama from making them kill themself. Napping is spoken of the same way we talk about bungee jumping- superficially fulfilling, but really really dangerous and not something most people do.
People are legally required to not live by themselves (so as to have someone do the nightly bondage stuff), and the homeless die via llama induced suicide a lot.
