I like the idea of magic being a glass cannon in games. Really powerful, but fickle.
The trick is making it incentivized for players enough to be worthwhile, while also actually posing some risk. Of course the trade off is that this would apply to villains as well. While probably anti-climactic to have Vecna melt his own skull off, it just might help out mid fight.
A big chunk of the inspiration for this was the book of the dead from Evil Dead. Where just reading the book (or playing the audio recording of it being read…) was enough to invoke the ancient evil, even if the actual speaker had no idea what they were doing.
-Anyone can cast a spell of any level they have encountered. Most are written in some esoteric tongue (i.e. latin, etc. ) but other than that they are accessible. No one wants to actually use a book that was written in code that they’ll have to decipher every time they want to use it. Magic has its drawbacks though. Every time you cast a spell there is a chance of something bad* happening. The very act of learning magic also corrupts the mind and soul distancing you from the people around you. Every spellbook you read will pervert your sense of reality in some way (this is the true reason why wizards are running around wearing pointy hats with stars on them or live as unwashed hermits and so forth) [mechanic effect: for every 6 spells learned, a quirk is developed. It may be assigned from a spellbook (GM) or if you pick up 6 spells a la carte from scrolls, self study or what have you, you get to make one up!]
So why be a magic user then?
Magic users don’t suffer the chance of something bad happening when they cast spells within their class/level restrictions.
Magic users can also create spells through independent research. Non-magic users can only cast spells they come across in books, scrolls, or are taught by other means.
*So what do I mean by bad stuff.
For any spell cast above the normal class level ability for spell per day / level make a saving throw vs. Magic. Upon failure roll on the following table.
If you fail the saving throw here is the spell fail effect. (roll 1d20)
1 Overcome by otherworldly visions collapse for 20+d20 hour coma. When you wake up you understand more of the deeper functions of the universe. Ask the DM one Yes / No question.
2 1d10 wiz damage 1d10 int damage. Heals at the rate of 1 point an hour
3 cast random other spell of the same level 1-3 cleric spell 4-6 magic-user spell
4 spell failure, no other effect
5 Reverse the spells effect (heal causes damage etc)
6 Mana burn: take 10% max HP (round up) as damage. Spell fails. If this kills the caster they burn up from the inside leaving nothing but ash behind.
7 Spell fails. It was another magic user that made you fail, you are sure of it. Probably that friend of yours. They’re jealous of you and your power, best keep you eye on them-the probably want to steal all your stuff for themselves.
8 Cast Summon (See LotFP Rules and Magic, its free - google it)
9 spell goes horribly wrong (like the transporter incident from Star Trek 1, let your imagination go wild)
10 Illusionary spell: Caster thinks it cast successfully, no-one else sees anything
11 Change target randomly (spell caster included)
12 Area of effect targets single target (closest to center) single target spell affects 1d6 1-2 sphere 3-4 cone 5-6 line (centered on original target)
13 magic drain, suffer -1 int or wiz for # turns = spell level, spell cast as normal
14 stunned -1 to all rolls for # of rounds = spell level, spell cast as normal
15 spell cast with minimum effect, all dice are 1s
16 spell cast with altered cosmetic effect, mechanical effects are the same. Lighting damage is instead giant balls of hail, or thorny vines that whip out and attack victim. Sleep is extreme apathy that makes someone completely unresponsive, prismatic spray shoots bubbles, or butterflys, etc.
17 spell cast as normal
18 spell cast as normal
19 spell cast as normal
20 Spell cast successfully with overpowering effect (max range, have damage dice explode, etc. etc. sleep causes coma, charm creates obsessive sycophant, etc. )
For: OSR/DND 1-2nd ed
Saving throws favor wizards and give ¼ chance of success at 1st level and get to roughly ¾ chance of success at 20th level. I like this margin because it isn’t horrible at 1st level, but there is still risk even at 20th level. Other classes don’t have quite such good odds, but that’s ok, they aren’t magic users, so I don’t mind there being more risk for them.
For 3rd ed/Pathfinder I think a DC 20 save. It makes the base difficulty a bit higher, but with attribute bonuses and feats I can see it also being possibly abused.
The 15-20 results are so when a spell is cast- given a ¼ chance of success on the save. There is a ~7/16 chance of the spell being cast successfully and a 9/16 chance of something going wrong. Risky but not unreasonable odds I think.
I was tempted to include hp damage to the list, but realized this just unfairly punished magic users compared to fighters etc.
A future project, may be to revise the list to include spell specific side effects. But that is a project for another day.