In this case, about magic, and magical fatigue. Basically, I've said many times before that in the Rubyverse, there's dozens upon dozens upon dozens of magical systems, but most of them more or less draw from the same basic forces. Essentially, there's nearly-infinite amounts of energy when you go digging into different sources, but no mage, not even Master Azev (one of Ruby's rogues, who is basically a villainous version of Doctor Strange), has absolutely infinite magic.
An example I want to talk about is the Elemental Eight Riders. They have a rather large magical reserve, one which differs from Rider to Rider, but which is always vast. D.D. and Hannah have the largest natural amount, though Whitney's efficient usage of what she has places her on their caliber. Ruby and Amy both have their conflicting powers (both being creatures of light and darkness) give them an immense boost, but that's not infinite.
Vili also has a lot of power, but hers mainly comes from the technique of drawing magical energy from surrounding sources. This is the technique I wanted to talk about. Almost universally, magic works on the principle that the user has a certain natural amount of magic in their body, but the ability to draw in extra magic from other sources. Some magic systems have virtually zero body-magic (thus, rendering using it akin to Cast From Hitpoints levels of dangerous) and are nearly-entirely based around drawing things in from whatever fuel the magic uses, while other magic systems have it almost entirely come from the user themselves with any drawing from others being borderline black/forbidden magicks.
The Riders fall in about the middle, overall, where they hold this immense natural reserve yet are perfectly capable of drawing from the environment to fuel their magic. The thing is, though...there's something near-universal about drawing magic into the body: it's taxing. No matter what the magic is being used for, and no matter what the source of the magic is, that tax is existent in every form of magic in existence in some form or another.
It manifests differently in each of them, but it's an unwritten magical rule: over time, stress builds. This is what I referred to when I was referring to magical fatigue. Blood Masters, the example I used it for, can live for centuries, via the mastery of the magic in their blood. But that magic exudes a slight tax, no matter how minuscule, on them, and as they continue to age, the more magic they have to use to maintain themselves and therefore the more stress they're placing on themselves. In short, the tax grows with age. And, eventually, after centuries, they simply can't take the fatigue anymore, and their body fails.
Magical fatigue is not often lethal, though. Magical fatigue is basically the body rejecting the magic, in an effort to maintain itself: the magic would kill them if it was used any more, so the body cuts itself off from the magic to prevent that from happening. So the only times magical fatigue kills are when the body needed that magic to live (as in, a no-win scenario where magic kills them and lack of magic kills them), or the magical fatigue is overridden and the person decides to use the magic anyway.
A far more benign case of magical fatigue can be found in D.D. and Whitney when they synchronize their abilities. Because they can both break the second rule of thermodynamics, essentially, by touching each other, Whitney gains free ice and D.D. gains free fire, giving them new magical reserves of energy each time they do it.
However, this process involves them creating/drawing in new magic rather than using their own reserves of magic (the whole point of it being to replenish their reserves), and so, magical fatigue begins to build up each time they use it. If they use it too much, then their bodies will literally collapse, giving out on them and having them pass out from exhaustion.
Soyeah. That's magical fatigue. I think it's a pretty neat concept myself, and I'm glad I had this chance to blog about it. Totally would have remained an obscure background detail if I hadn't had the time to think about it and blog about it just now.
So, uh. Go me, I guess!