When we consider the many magic spells in the Slayers world, we sometimes stray into a complicated maze. Yes, it would be easy to just leave it as "it's a mysterious power", but we want to look at it systematically. Here, we examine the system and process of the magic in Slayers. It's complicated and sometimes difficult to understand. Most of this is drawn from the novels, but some of it is my own speculation on the subject, done in an attempt to make a single "manual" for the dictionary. Now, let us examine the basic knowledge of magic.
What is 'mahou' (magic)? Before we understand that, we have to ask, what is 'ma' (evil)? Everybody knows some simple natural laws. For example, "everything drops". Of course, this is due to gravity. It's why people can't fly... when we jump up, gravity pulls us back down.
But what would happen if we could make another law?
What if we wanted to fly? We could make another law... "objects in motion tend to stay at rest" instead of "everything drops". Now, gravity doesn't pull you down, and you can fly. But to fly freely is another thing. If you want to do that, you need to add more new laws.
This new law and the energy that should not exist naturally, is 'ma'. 'Mahou', then, is 'hou' (skill) in how to use 'ma', the unnatural laws and energy, by spells.
Where does the 'ma' come from, then? Strictly speaking, the 'ma' doesn't exist in a place. 'Ma' is created when a spell is cast, overwriting one set of natural laws with another, which will be restored when the spell is ended. This is not surprising... 'ma' is always against the natural laws. In other words, 'mahou' (the magic) temporarily changes the natural laws in a way that makes something happen. The bigger the change in natural laws, the longer the necessary incantation of the magic spell has to be. It makes sense... when we want very unnatural things to happen, we have to change more parts of the original natural law.
Magic comes to happen when the magic user interferes with the Astral Plane, which coexists with our Material Plane. The magician channels certain energies from the Astral Plane. Taking the raw energy of the Astral Plane and applying it to unnnatural law that is made by a spell, the magic user produces... well... magic.
Most of the Shamanist spells convert this magic into physical substance, like a Fireball. Others, like the Elemekia Lance, do not convert the energy. This is important... remember that Mazoku are invulnerable to physical attacks, so only the latter category can harm them.
Every magic spell requires a minimum amount of magical energy. If the caster does not have that amount of magical energy, then he or she cannot cast the spell - simple, right? Not exactly.
First, the magic energy is finite, so a spellcaster can't cast spells indefinitely. The energy recovers when the magician rests, though.
Also, it is not possible to blow your entire magical capacity in a single spell. Not only is the magician limited in how much his maximum magial capacity is, he is also limited in how much of that magic he can use at one time. This second limitation is called "maximum magical power". It's like a city with one freeway. No matter how many cars are in the city, only so many cars can come out at any one time.
Therefore, if a magician doesn't have enough magical power to cast a spell, he or she can -never- cast it. If another magician has enough magical power to cast the spell, but not enough magical energy (say it's at the end of a long day), then that magician is also unable to cast the spell. Only when a magic user has both enough magical power and enough magical energy remaining can that magic user cast a spell.
We know that Dragon Slave needs the largest maximum magical energy that one human can have. We also know that the sorcerer/sorceress can cast several Dragon Slaves one after another. Now you know why successive Dragon Slaves are possible.
People can't see raw magic moving around. It's invisible, like ghosts or spirits. However, we can see various light produced by that magic, like the red light that gathers when somebody casts a Dragon Slave. This is something like a reflection of the energy in the Astral Plane. This light is not the actual energy, though. Of course, spells like Flare Arrow make real fire, and that creates light as well.
Chaos Words are the language of magic. They are used in the incantations that form magic spells. However, because the Chaos Words are totally unlike any of the languages spoken in the world of Slayers, it is very difficult to master them. The use of Chaos Words is both the "key" and the "plan" for magic. The caster chants the words, both opening the door to the astral plane for the energies to get through, and making those energies behave to form the correct result. Because several spells have very similar effects, they use similar Chaos Words. For example, many of the fire spells have a similar opening to their incantation.