I think “Feast of Souls” by C.S. Friedman fills the bill nicely. From my review of the book:
This is not a spoiler, it’s part of the basic setup of the plot. BTW, this is not an “eventually” thing: witches rarely live beyond age 30.
Some of the answers involve magic being extremely dangerous when something goes wrong or when used incorrectly. Do those instances qualify? Because almost all fantasies involving magic have that particular trope.
Avada kedavra , pretty much the definition of dangerous. To the target, of course.
‘Beyond the Sea Gate of the Scholar Pirates of Sarskoe’, ‘Sir Hereward and Mr. Fitz Go To War Again’, and ‘A Suitable Present for a Sorcerous Puppet’, short stories written by Garth Nix. The effects of magic on people aren’t what the stories are about, but you still don’t want to mess with the esoteric. And they’re cool little stories too.
The magic system of Steven Erikson’s Malazan Book of the Fallen series is very dangerous if you don’t have sufficient control. The warrens can actually blow right through an insufficiently strong mage, and the use of chaos is directly deleterious to the health of normal-world beings.
The Abhorsen series has some pretty dire dangers for failure to use magic properly, too. You don’t want to just go around ringing those bells.
In L E Modesitt’s “Recluce” series, there are two kinds of magic…Order Magic, which is fundamentally constructive, and Chaos Magic, which is fundamentally destructive. Chaos Magic, unless the Chaos Wizard is extremely careful, tends to prematurely age and weaken the user, and a lot of Chaos Wizards, especially later on in the timeline of the series, tend to be pretty mentally unstable and megalomaniac.
Order Magic tends to be less dangerous to the user, but it is shown that use of powerful Order Magic, especially to harm others, can lead to the Mage being struck blind or otherwise disabled, sometimes temporarily, sometimes permanently.
You might try the Night of Madness and **The Unwilling Warlord **novels by Lawrence Watt-Evans. His Ethshar world has multiple types of magic, but the warlock version is pretty much my definition going to result in a bad end.
Thought of another one: In David Weber’s Bahzell Bahnakson books (starts with Oath of Swords), magic isn’t inherently harmful, and there are some forms of magic which are safe… but the sort of magic used by wizards inherently requires expenditure of life-force to get any significant effect. The good wizards would use their own life-force (including complete self-immolation in extreme necessity) or that of willing volunteers, but they’re now extinct, with the only wizards left being the evil ones who gain power by sacrificing others.
The Dragon Waiting by John M. Ford fits. It’s an alternate history of Richard III with magic. Magic almost always systematically destroys the user over a long period of time, which is used as a plot point; there’s one gruesome scene where an example of this is shown. (There are also vampires, where vampirism is treated somewhat like heroin addiction (if heroin addiction had the side effect of immortality). The vampires are also essential to the plot and the construction of the alternate history (which forks with Julian the Apostate and Justinian). I almost never like vampires and I like the way he does vampires.)
I find that John M. Ford is something of a specific taste – he’s very dense and allusive and often difficult to follow, but man, this book is so brilliant.
Sorta-kinda what you want: Rick Cook’s “Wizardy” series. It’s not so much that magic is inherently bad, it’s that this is a super-high magic world. Imagine walking around in a suit made of flint and steel in an atmosphere that’s got a ton of oxygen and hydrogen.
Our hero (a programmer from this world) says “So…wait. You’re saying that magic works? Like” he grabs a twig off the ground, twiddles it in a few circles while saying “this? BIBBIDY BOBBIDY BOO” and three HUGE fireballs shoot out, nearly setting the entire forest alight. And it’s not really replicable. Each gesture, timing, inflection must be perfect and identical. If one new safe spell per century is created, that’s a GOOD century. Most people who do anything remotely magical without decades of training usually die horrible deaths.
In Duane Duane’s The Door into Fire and sequels there’s two types of magic, sorcery (which isn’t harmful), and another which is fueled by the life of its user and shortens their lifespan. Don’t be fooled by the embarrassingly bad (and inaccurate) cover, it’s good.
In L.E. Modesitt Jr’s Spellsong Cycle books, “Darksong” is sorcery that affects living and once living things; it is progressively dangerous to sorcerers. It’s compared to an allergy, where the more exposed you are to it the more sensitive you become. The effects are easily fatal.
In The Guardians of the Flame series magic is dangerous because it’s addictive. The more powerful a wizard is, the more obsessed they become with magic. The most powerful people by far in the setting are the great wizards - they are continent breakers in power - but they almost don’t matter in practice because they are almost completely focused on cramming as many spells as possible into their heads.
In The Diplomacy of Wolves and sequels magic deforms those who use it in an inhuman fashion, although many get around it by forcing the deformities onto other people.
Yes; it’s
the (stolen?) walking stick of the Mother of Winter, and allows him to safely use black magic.
In The Belgariad, the Will and the Word was generally not harmful, but trying to make something not exist was immediately fatal. Since this was a form of magic that could easily be learned accidentally, you had a one in three chance that you wouldn’t even know you could perform magic before it killed you.
Mages in the Dragon Age games are pretty much inherently dangerous to themselves and anyone else. Anyone with the gene for magical talent, whether or not they have any training, is constantly at risk of demonic possession, especially while asleep (since the mind passes into the spirit realm while dreaming). A single possessed mage can easily bring dozens or hundreds more demons through into the physical world with them and create mass chaos, to the point that most cultures kill or imprison all mages and magically lobotomize the ones who lack enough skill to defend themselves against possession.
Terry Pratchett frequently touches on this, but it’s most central to Equal Rites.
The Warhammer world has a concept that magic is the use of chaos. If you are evil or chaotic, then the magic does literally harm you over time (physical ailments, insanity, etc. or even turning you into a demon over time), though they’re nice enough that those who are good or neutral don’t suffer the same effects.
I haven’t read enough of their fiction to know whether 1) this is still true (Warhammer is notorious for re-inventing things) or 2) they actually use this trope much in the fiction.
I’m surprised no one has mentioned HP Lovecraft yet. 
It’s not so much that “if you’re evil, magic will do bad shit” and more “one third of the magic in the world taps into the very real and very fickle embodiments of chaos/evil”. So, yeah, that’ll leave a mark. And that’s before those embodiments actually notice you, start taking an active interest and just give you eternal life with supercancer or randomly mutate you into horrible shit because that joke never gets old.
One other third is powered by magic plutonium, so there’s that too.
And then there’s Orky magic, which is less “subtly corrupting” and more “if I don’t discharge my accumulated magic RIGHT NOW my head’s going to explode. Literally. Oh, and the more excited you (ork) guys get, the more uncontrollable magic flows into me. Oh, you guys are really excited about me desperately flinging lightning bolts about in order to NOT DIE. Awesome”.
That being said, I’m not sure how much of that is explored in the novels, which are typically more sword than sorcery. “Chaos magic is chaotic, Chaos gods are dicks” is explored more thoroughly in the Warhammer 40.000 novels.
As I recall, demonology (which was using the Will to create monsters) was pretty dicey too. As in, you lose control of your pet “demon” for half a breath and suddenly* Saw*.
Yes. Demons in that world were controlled by forcing them into a body that the summoner has created. The more detail involved in that body, the greater your control of the demon. If you leave out things like blood, if they’re injured, your control weakens. Summoning demons in their natural form into the world is a definite no-no, and doesn’t typically end well for the summoner. At all.
I absolutely adore LWE’s Ethshar books, which is odd as I rank most fantasy as below Ikea assembly instructions on my interest scale.
Warlockry has a direct and almost unavoidable cost, but all of his schools of magic take some toll and have distinct lethal pitfalls - even the relatively benign witches can exhaust themselves to death. (It’s possible that theurgists have no lethal consequences, although they could screw up their afterlives.)
He never has fully explored what those “scientists” do, though. 