(I'm never being ambitious again, I swear... Goddamn scope creep... Also I am not rhyming.)
In AU1, MP is another resource in an addition to HP and TP.
MP is tied to using Spells and Bullets, and the Magic stat determines how effectively MP can be used.
MP is Darkner exclusive resource. Most Lightners can use their large well of determination as a substitute to MP to use Spells and Bullets.
In addition to enabling use of Spells and Bullets, just existing as a Darkner and doing stuff takes MP. Just like it takes energy for Lightners to exist and do stuff.
If MP is too low, the Darkner in question becomes exhausted. This does not affect their HP, but they become more vulnerable to things that can deplete their HP.
A Darkner cannot die - meaning being irretrievably lose their consciousness - from having too little MP. If they gain MP again, no matter how long they have had very little MP, they will wake up and be able to think and feel again.
When resting, a Darkner slowly regains MP. This is an ineffective way to regain MP, and can lead to Darkners leading a very inactive life, so many Darkners turn to other ways to gain MP.
Eating is one way to gain MP. Different things are edible to different kinds of Darkners, for example Addisons can eat currency and Worm/Coody can eat leaves.
MP can be shared from one Darkner to another through healing Spells or consumables made by use of Bullets or Spells.
Darkners can also gather MP through performing certain actions, being interracted with in certain ways or absorbing it from their surroundings. These ways of gathering MP are related to their purpose and different species of Darkners have differrent ways of gathering MP. For example, Darkners whose Light World counterpart are powered by electricity often gain MP just through their object being plugged in, while Organikk and Vicabell can gather MP through actively making music.
When a Dark Fountain connects the Dark World to the Light World, Darkners gain MP more effectively through resting. In addition, it cost less MP to use Spells and Bullets.
There are two basic distinctions in uses of MP. Bullets and Spells.
Spells are basically all things that use MP to do things that aren't Bullets.
For the most basic example of Spells, see the options* under the MAGIC command in battles. Ralsei's Heal Prayer and Pacify are classics! Susie's Better Healing and Noelle's Sleep Mist are other examples, as well as the upgraded DualHeal under the ACT command.
*(except S-Action, R-Action, N-Action and some others)
The distinction between Spell and Bullet is loose at times, but in many edge cases it's a Bullet used to channel a Spell. A green Swatchling's Bonbon Bullet and Spamton's [Press F1 for Help] are examples of a healing Spell being channeled through a bullet. King's Swing attack is another case of a Spell being channeled through a Bullet, but not a healing spell this time.
Spells can also be used outside of battle. Examples of this include Roulx's teleportation and Tenna's Scene Change spell, that he uses to bring Kris, Ralsei and Susie into the Green Room after rounds or into the big toy balls after discovering them by Toriel. Spells to make Items are also used outside of battles, since they often take time to cast.
Spells cannot only be channeled through Bullets, but also through Items, like Queen channeling her Firewall spell through her wine glass or Noelle channeling Snowgrave through the ThornRing.
Some spells can also cause status conditions. Gerson and Sound of Justice have used spells to turn the SOUL green. I do not currently know if the Constricting Darkness and yellow SOUL status conditions are spells or not.
Bullets are MP manifested into physical energy outside of oneself in a way that can lower the HP of someone.
This includes most classic Bullets, but also Rude Buster and things that traditionally aren't called Bullets, like the Knight's slashes that disarm Tenna and in their Box Splitter attack.
When channeling a spell through a Bullet, the Bullet can also be manifested in such a way that they don't lower HP. There are other instances of Bullets not lowering HP as well.
Bullets can also be put into two basic categories. Basal Bullets and Zooid Bullets.
Basal Bullets are, like the name suggests, the basic type of Bullet. MP is just shaped into a Bullet by the Darkner, nothing more nothing less. They can have different shapes, patterns of moving, elements and damage outputs.
Zooid Bullets are a bit more complicated, as in addition to MP the Darkner also puts a part of themselves into the Bullet. This enables more advanced Bullet patterns, as the Darkner who made them often has a greater degree of control of the Zooid Bullet and/or can channel Spells more effectively through them.
Zooid Bullets can also make Basal Bullets of their own, and even rarer cases, make additional Zooid Bullets.
The easiest way to tell Basal Bullets and Zooid Bullets apart is that Zooid Bullets often resemble the Darkner a lot more than Basal Bullets and/or have a face. (But there are outliers, like the Ads of the Addisons which more closely resemble their Light World counterpart rather than their Darkner form.)
Different Darkners put different parts of themselves in Zooid Bullets, and Zooid Bullets can often represent parts or characteristics of the Darkners Light World counterpart to different degrees.
For example, Rabbicks Floaty Rabbit Bullets have physical part of their Darkner forms in them, and represent the way their Light World counterpart moves.
In the opposite side of the spectrum, Maus' Critter Bullets contain their sense of restlessness, and using these bullets allows them to express it in a way that helps to remain in battle when they would rather flee or zip off to do something else.
The Darkner who makes a Zooid Bullet can sense through all of their Zooid Bullets, but to different degrees in every species. For example Ribbick's can smell and see through their Frog Bullets. Individuals of the same species of Darkner can also be better or worse at sensing through their Zooid bullets, it's a honed skill.
Zooid Bullets can also be stored for later in special inventories Darkners have for this reason. It's a way to not have to spend as much MP on attacks in battles.
Some also use it as a way to store MP outside of the MP bar, since prepared Zooid Bullets can be turned back into MP. Though unlike food that restores MP, the MP gained by turning Zooid Bullets back into MP cannot be shared with other Darkners. Except if the Darkner who partakes in the MP is of the same species as the Darkner who made the Zooid Bullet.
Darkners that use Zooid Bullets a lot and outside of Bullet battles - for example to gather MP or other common tasks - can be regarded as a kind of communal organism, where the Zooid Bullets can be regarded as "organs outside of the Darkners body".
That is why the term Central Darkner is used. It is to distinguish the Zooid Bullets from the "actual body" of the Darkner - the part that houses the Darkners Inventory and central consciousness - termed Central Darkner.
Zooid Bullets can be a large part of how certain Darkners experience and process the world, their thoughts and emotions; and can therefore a big part of their identity. This of course this differs from individual to individual. But to respect that in terms of language, the term Darkner includes both Zooid Bullets and the Central Darkner in its common usage. The distinction is made when necessary.
