How to cast a spell

When I was GMing 2nd and 3rd edition D&D I had this houserule that wizards didn't have to memorize spells every day but they did have to roll a skill to cast a spell. In 2nd edition it was a proficiency and in 3rd edition it was a skill, but both were called Spellcraft.

At first it was just a simple roll to see if the spell went off, a wizard PC would roll a d20 and add their Intelligence modifier. I rated the level of the spell as the difficulty, added to 10, so a 1st level spell needed an 11 or higher to cast and a 19th level spell needed a 19 or higher. Fail the roll and the spell fails, but you didn't spend the spell slot either.

During 3rd edition, I changed it to be more similar to combat. Casting a spell had a DC of 20 and the PC could add both their Spellcraft skill and their Intelligence modifier to their roll. Failing a roll still meant the spell didn't go off and you just had to try again.

I only ever had one player walk out of a game because of this houserule because anybody who played for more than one session learned about my other two houserules for wizards.

#2: Spell slots were directly translated to spell levels and could be spent like points to cast spells, so a 5th level wizard would have 10 spell levels to cast spells with while a rules-as-written 5th level wizard would have three 1st, two 2nd, and one 3rd level spell slots.

#3: Wizards could cast any level of spell as long as they had learned it and had enough spell levels to cast it, during 3rd edition days the DC for learning was based on spell level but during 2nd edition days it was a percentage roll based off Intelligence.

I forget how I adjusted 0-level spells, but players rarely used them anyway.

All of these together made wizards a lot more powerful earlier on, and it also freed the typical wizard player from focusing all of their time on strategizing their spell lists. Something else happened, during later sessions other PCs started thinking about picking up a level or two in wizard. Which made me start thinking "why do I even need a wizard class? couldn't I just let anybody with Spellcraft attempt to cast a spell?" But then I stopped GMing and sat on the player's side of the screen for the next decade or so.

Enter: World of Darkness

When I returned to GMing a longer campaign I attempted to splice my favorite setting of Birthright into the rules of World of Darkness. Instead of using the Mage rules straight from the book, I picked apart spells and applied them to the D&D tropes of spells. I had a short list of available spells that were detailed from top to bottom. And all of that was a mistake. Often my players cast spells expecting them to work just like their D&D counterparts, even though I wasn't trying to emulate them exactly but marry them to the low magic and gritty feel I was going for, or they simply forgot they had the spells.
World of Darkness combat is also a slog so many optional and house rules were introduced, but this is a different subject for another time.

Enter: Dungeon Crawl Classics

I really enjoy this system. It feeds the nostalgia of my days playing and learning the rules of 1st edition AD&D, but it's rules-light emphasis with 3rd edition mechanics means it is also very easy to learn and utilize and build upon. It's also a system that requires a wizard to roll a d20 to cast their spell. The only thing I didn't like about it was that the spells were sometimes written across 3 or 4 pages. Half of the rulebook is literally the spells many varied results, and I couldn't help but feel there is a way to simplify them, or there should be.

Enter: Apocalypse World

We played a short campaign of this and it changed the way I view role-playing games. The mechanics of the partial success are simple and evocative and can apply to anything. But the rulebook offers ideas for applying them to your favorite game. From page 279
Magic User: Cast a spell (intelligence)
Arcane magic comes from the use of formulae, ritual, and the magic user’s own life force.
Roll 7–9: Player chooses 1
Roll 10+: Player chooses 2
• the spell is not forgotten
• the spell has a powerful effect (maximize dice)
• the spell has a large effect (double range, duration, or number
affected)
• the spell does not misfire
The implication being that if you roll a 10+ and choose for the spell to have a powerful effect and it doesn't misfire, that the spell also doesn't have a large effect and you forget the spell.

Enter: OSR influences

In a low magic, dark fantasy setting, where I want ability to take precedence in defining characters, how do I apply these lessons to make spellcasting costly but powerful and evocative of old school randomness but simple to use without charts or levels?

Blood magic. Power the spell with your blood or somebody else's, roll 2d6 and add Magic.
Roll 12+: success
Roll 8-11: success, but choose 1
Miss: all 3 are true
• the spell is lost until you rest
• suffer a cumulative -1 to Magic until you rest
• the spell fails or is cast and misfires, Oracle's call

Identify
Allows the caster to know all of the magical properties of a touched magical item.
Cost: 1 hit point, +1 hit point for +1 to Magic roll

Invisibility
Turns the caster invisible to sight until they interact physically with another object or person.
Cost: 2 hit points, +1 hit point for +1 to Magic roll, +2 hit points for the invisibility to last until dismissed (or loss of consciousness)

Fireball
A magical energy bolt flies from the caster's hand and hits another living target, causing 1d6 magical damage.
Cost: 3 hit points, +1 hit point for +1 to Magic roll, +1 hit point for +1 damage, +3 hit points for +1d6 damage

Teleport
Instantly transports caster to a location they have been to before.
Cost: 4 hit points, +1 hit point for +1 to Magic roll, +4 hit points to bring another living creature and what it's carrying

Rune magic. Take the time to draw the rune with the appropriate ink, roll 2d6 and add Magic.
Roll 12+: success, choose none
Roll 8-11: success, but choose 1
Miss: all 3 are true
• working carefully, it takes twice as long to draw the rune
• your work is sloppy, use twice as much ink to draw the rune
• the rune is drawn wrong, it either works strangely or it works in a very bad way

A typical vial of ink holds 4 uses. A typical bottle of ink holds 12 uses. An entire jug would hold 36 uses.

Trigger
When another living creature steps over, or passes by, this rune the caster instantly knows.
Cost: 5 minutes and 1 use of ink

Alarm
When this rune is activated by a Trigger rune it makes a loud noise, determined by the writing of the rune.
Cost: 5 minutes and 1 use of ink, +10 minutes and +1 use of ink for the sound to come from somewhere other than the rune, +5 minutes and 1 use of ink to designate a person to hear the sound regardless of distance from the rune

Wall of Fire
This rune creates a wall of flames that does 1d12 damage per round. These flames must be set by a Trigger and last indefinitely or until the rune is broken, unless another Trigger is applied to cease the flames.
Cost: 4 hours and 6 uses of ink, +4 hours and 6 uses of ink for +1d12 damage per round

Protection from Heat
This rune protects the object it is written on from fire, flame, and heat. Completely.
Cost: 10 hours and 20 uses of ink

Tattooing a rune onto somebody's body always adds +4 hours and +8 uses of ink to the Cost.

These are just ideas at the moment. I'm going to flesh out Blood Magic a bit more and make it less D&D-y, but I wanted to work out how some spells I know would work in such a system before I start making really unique pain-worshiping blood throwers.