This is a magic system I have been testing. It is general enough that I think it should be easy to use with other traditional fantasy games, if you like, given a small amount of adjustment to the system you are using.
This is a roll to cast system. You need to decide on a set of known or prepared spells for a character. Then, the die outcome will take care of the resource aspect of spells. You also need a resolution system that provides five levels of outcome. You can think about these outcomes as levels of player character success or as augury into the fictional world. The five outcomes are:
- Principle intensified
- Principle & corollary
- Corollary & decline
- Catastrophe & decline
The principle is the basic spell effect, the thing that the sorcerer is generally attempting to bring about. The principle intensified is the critical hit version of this. The corollary is an unintended side effect that generally complicates the magician’s life but may occasionally be useful. Decline represents a decrease in future spell potential or magical energy. The simplest implementation of decline is, following a Vancian approach, forgetting the spell. And finally, catastrophe is a total misfire which may have personally deleterious effects. This set of outcomes means that something magical always happens but the potential corollaries and catastrophes should make players think twice about playing with arcane fire.
To resolve the casting of a spell, I use an ability check, similar to the approach described here, with intelligence for black magic and wisdom for white magic.
For an approach closer to classic d20, consider the following:
Let target = 10 + spell level. Then, roll 1d20 and add spell bonus (something like one-half level, rounded up). Interpret the result as follows:
- Natural 20: principle intensified
- Target + 4: principle
- Target: principle & corollary
- Below target: corollary & decline
- Natural 1: catastrophe
Further, you need a way to determine what each class of outcome means for a given spell. Writing new spells with extensive corollary and catastrophe tables is wonderful but by no means necessary.
Ideally, catastrophes are specific to individual spells, but lacking specific catastrophes, you can fall back to some general outcomes, such as these:
- Wonder & Wickedness: spell catastrophes
- Vaginas are Magic: miscast table (inside front cover)
- Last Gasp Grimoire: Do not take me for some turner of cheap tricks
- Basic Red: Wild sorcerer effects table for 5e
(I am sure there are others available too.)
Hacking a spell to support this system can be easy. For example, here’s a hack for the classic spell magic missile:
- Principle intensified: +1 extra missile
- Principle: as written
- Corollary: an additional missile targets something non-animate (determine randomly)
- Catastrophe: determine target of each missile randomly (friends and foes)
For many spells, it is feasible to rule on potential corollaries and catastrophes in real time. I would just clarify with the player what the potential scope of outcomes would be and confirm intention to cast the spell prior to the roll.