"When the mana runs out, I'll go like a blown candle flame, and civilization will follow. No more magic, no more magic-based industries. Then the whole world will be barbarian until men learn a new way to coerce nature, and the swordsmen, the damned stupid swordsmen, will win after all."
Larry Niven's The Magic Goes Away stories tell of an ancient civilization based on Functional Magic powered by "Mana", but there's only a finite amount present on Earth. That nobody seems to be aware of or acknowledge this fact causes the magi, magical creatures and gods that use mana to eventually "go mythical" (a very obvious allegory aimed at modern civilization's reliance on fixed resources).The device that first proves this effect, and later becomes the most dangerous weapon in the world, is the "Warlock's Wheel", a simple copper disk with two spells on it: one that makes it spin ever faster without limit, and a second spell that makes the disc indestructible so long as there is magic available. Small versions of the device can be used to block scrying by making a wall of magic-free areas around a fortification, but a sufficiently large one could ruin an entire nation. In later books, the Warlock's Wheel becomes a sort of Memetic Badass, as a mage refuses to even draw a picture of one because even a drawing of one would suck all the magic out of him.The short stories and novels include:
Not Long Before the End (1969) — the short story that started it all
Unfinished Story #1 (1970)
What Good Is a Glass Dagger? (1972)
The Magic Goes Away (1976, expanded into a novlet in 1978).
And I Must Scream: Wavyhill made the mistake of casting a spell to keep himself alive. Even when his body is destroyed. He spends a few decades as a skull before Warlock gives him the ability to talk again (Though he's still just a skull).
Anti-Magic: The Warlock Wheel. It's a simple bronze wheel with two spells on it; one to make it spin faster, accelerating without limit. The other spell prevents it from destroying itself. This wheel rapidly eats all the mana in the area, turning it into a dead zone where magic no longer functions at all.
The rest of the world is slowly becoming like this, as mana is a local, finite resource. Using magic will slowly deplete an area, until it becomes a dead zone.
Glirendree stops hostile magic from reaching its wielder, which is part of what makes it so effective. You know, until it kills you.
Author Tract: It'd be less blunt for Niven to elbow you in the ribs and scream "OIL! OIL, YOU GET IT?!?"
Awesome, but Impractical: Old-style magical feats, like flying dream-castles made of clouds, are considered both tacky and terrifyingly wasteful compared to modern spellcasting, which emphasizes efficiency.
And once it's found that mana is finite, they're considered too dangerous to keep using for most anyone who has two brain cells to rub together, since the instant the magic runs out, you're dead.
Artifact of Doom / Clingy MacGuffin: Glirendree the demon sword. Using it is a bad idea. On one hand, you're nigh-unstoppable and mostly immune to magic. On the other hand, it'll consume your life force and leave you dead of old age in a year or two, and you can't let go of it. Warlock reverts it to its demon form and feeds it to the Wheel.
Black Magic: "There's mana in murder." Black Magic used to be too dangerous to use at all, until mana began dropping. Wavyhill the Necromancer develops a system for using murder-mana safely, inventing necromancy. By which we mean he murders boatloads of people.
Book Ends: The first story in the 'verse, "Not Long Before the End," starts by mentioning that swordsmen and wizards often fight, and usually the wizards smear the swordsmen across the map, thusly improving the human race by removing an idiot (And rarely, the wizard loses, and that's a net improvement too. A wizard that can't beat a swordsman is a poor wizard.) The story ends with the page quote, a sad reflection that the swordsmen are ultimately the winners, being unconstrained by the loss of magic.
Boring, but Practical: Warlock versus a barbarian with a demon-sword results in the Superweapon Surprise, but Warlock is left in a withered body several hundred years old and without magic, while the barbarian merely lost his hand and the sword. The barbarian tries to finish him, and Warlock pulls an entirely nonmagical dagger and kills the barbarian.
Colony Drop: During "The Magic Goes Away" novel, the heroes are hoping to revive a god and have him drop the moon on the Earth to provide a new source of magic. The heroes have no idea how large the moon actually is, and freak out when they realize that doing it will utterly destroy the world and leave the god as the only survivor. The god has absolutely no problem with this, thinking it's wonderful idea. They're forced to kill said god and stop the drop.
Compensating for Something: This is the only reason a magician carries a sword (Having far better options for actually killing things). Because of their phallic implications, wearing a sword is a cure for impotence... but you have to wear it in bed. Wavyhill wears a sword all the time, which Warlock finds riotously funny. It turns out the sword is enchanted to fight independently of the user, and he keeps it around to surprise Warlock. He knows Warlock will overlook it.
Functional Magic: Magic has specific rules, but the rules seem to run partially off belief. The moon is magical because "everyone knows that". Strong emotional reactions can generate mana, but this is largely limited to mana being generated by murder (But not war, which is different). There's rules for making spells and whatnot, but they're not heavily explained.
Gone Horribly Right: Athens' conquest of Atlantis. Athens had been jealous of the far more advanced Atlantis' wealth, and launch an invasion which kills the dynasty of sorcerer-kings that had been secretly keeping the island tectonically stable via magic. After several days of happy looting and pillaging, the Athenians (and Atlantis survivors) are rudely surprised by Atlantis sinking and killing everyone.
Here There Were Dragons: Lack of mana mutates mythical creatures offspring, if they're lucky. Dragons turn to stone or mutate into various things, unicorns are born without horns, giant slimes grow small, werewolves cease to be "were", and generally turn to "mundane" creatures. UNLUCKY mythical monsters simply keel over, going extinct. Merpeople, centaurs, and many others simply die off in mass extinctions.
I Know Your True Name: Knowing an entity's true name gives you a massive amount of control, even with weak spells. Wavyhill the Necromancer is revived as nothing but a skull thanks to knowing his true name (and a ill-advised spell he used to keep himself alive). Warlock's name-parent was smart and named him something that can't be pronounced by humans.
Wavyhill is annoyed when Orolandes manages to forget his real name despite being told twice, when a wizard would have given their left eye to know the true name of the first Necromancer.
Jackass Genie: Attempted, but subverted. Clubfoot knows full well the djinn will try to screw him over and takes it into account. The djinn's "twists" end up being what Clubfoot wanted, or at least close enough that he doesn't mind.
Literal Genie: The above djinn tries this too, and it has the same result.
Love Potion: There are love spells and loyalty spells, but they have the unfortunate side effect of robbing the victim's intelligence. Warlock hates them, saying that there's not a lot of point being surrounded by friendly idiots.
Ludd Was Right: Played with. Advanced, magic-based society will collapse utterly because of the vanishing mana, and the plots are largely how people are dealing with it. "Primitive" methods of doing things begin to work better, like constructing stout buildings to hold up their own weight instead of just using enchantments to keep them up. Or using swords to kill things instead of death-spells ("Those damn stupid swordsmen may win in the end after all"). Nothing is implied to have been wrong with magic-based civilization; no moral decay or anything. It's just that it was unsustainable in the end.
Meaningful Name: Since most mages are smart enough to keep their real name secret, they uses meaningful handles. Clubfoot is a native american with, well, a club foot. Warlock is an inversion; his name is just a nickname, but because he's so famous it eventually becomes a term for magic users in general after he's dead.
Originally Warlock took his name because he forced a region into peaceful co-existance by joining whatever side was not the aggressor, and was potent enough to tip the scales toward whoever he joined; as long as he was around, no country wanted to start the next war for fear of him. He called himself War-Lock, as in he "locked" the option of "war". By the time he's famous, the original reason for the name is all-but forgotten by everyone but him.
Otherworldly And Sexually Ambiguous: Roze-Kattee, the God of Love and Madness, is a hemaphrodite with a hideous appearance: a humanoid covered with shaggy, coarse hair, blazing yellow-white eyes brighter than daylight and pointed ears.
Phlebotinum Muncher: Dragons, Unicorns, and other mythical creatures require mana to sustain their metabolisms. Gods require truely ludicrous amounts, and mana deprivation renders them mostly mythical even during the ages of high magic.
Skunk Stripe: The witch Mirandee's hair has one, which is a sign her youth spell is failing from lack of mana. It gets bigger and smaller depending on the mana in the area. In mana-rich areas, it's small or not there at all. In dead zones, her hair goes completely white.
Superweapon Surprise: The Warlock Wheel was first intended to test a hypothesis the Warlock had about why his power seemed to fade over time, unless he moved to a new place. The wheel was considered a simple toy that revealed a terrible secret, but nothing more than that. Until later, when it was used to defeat an otherwise unstoppable demon, and then after that, a god.
The device is terribly simple: A simple copper disc with a pair of enchantments on it. One protects the disc from damage caused by wind resistance and centripetal force. The other makes the disc spin increasingly fast, with no upper limit. The disc spins non-stop, constantly accelerating beyond the point that it would ordinarily melt away or explode, until all the magic in the area is used up.
Wavyhill finds ways to improve it: He makes it directional, and of very precise scope. He uses this to create a dead zone around each of his homes, with a zig-zag path of non-dead zone through it. This makes his homes unscryable, as scrying only works in straight lines and doesn't work through a dead zone, but you can still go safely through it even if your life depends on magic, provided you can find the path, which is harder than it sounds.
The Time of Myths: Most of the stories happen at the end of an Age of Magic, some thousands of years ago. Thousands of years before that, Gods roamed the earth.
Weapon of Mass Destruction: The Warlock Wheel is treated as this. Warlock kept the Wheel secret for years because he was horrified at how destructive and easy to make it was. Every time it's used, the area goes magically dead, forever.
What Kind of Lame Power Is Heart, Anyway?: Terrifyingly inverted. Roze Kattee is a God of Love and Madness, and the heroes assume he'll relatively safe to deal with because those things are hard to leverage against them. But instead of doing the obvious and causing people to go nuts or fall for each other, he does the opposite; he can remove your love or irrationality. People effected find themselves sitting around, having lost their love of living or willingness to fight an obviously unstoppable god on the off-chance getting lucky. Also, he's still a world-warping godmonster, especially since he was revived by the death of the World Worm and has mana to spare.
When All You Have Is a Hammer: Orolandes the Athenean soldier, fighting Roze-Kattee, God of Love and Madness. Roze-Kattee stretches upward, tall enough to grab the moon and attempts to stop it in its orbit so it will slam into the Earth. Orolandes, having nothing more helpful to do, stabs Roze Kattee in the foot. This actually works out pretty well.