This is a system I've been using to allow magic-users and clerics to cast multiple spells. It's also designed to make magic more like it is in most fiction. From my reading it's more usual for magic to be useable indefinitely but dangerous. Whereas in the D&D system you can use a certain number of spells without any danger, but none beyond that.
The original version of this had instant death as one of the possible results on the Mishap Table. However I got several complaints that this was too harsh, and so changed it. From my experience in play, it's now too easy. So any suggestions are welcome.
These rules are inspired by articles in Fight On! issues 9 and 11.
Clerics and magic-users may attempt to cast unmemorised spells. The spells must be of a level and type that they could memorise. However level 1 Clerics may attempt to do this with level 1 Cleric spells.
To do so they first roll 1d6. The result depends on how many unmemorised spells they have cast today. Spells that fail and result in a roll on the Mishap Table count towards the total of unmemorized spells cast.
none: 1 roll on the Mishap Table 2-6 spell takes effect.
one or two: 1-2 roll on the Mishap Table 3-6 spell takes effect.
three or more: 1-3 roll on the Mishap Table 4-6 spell takes effect.
Roll 1d8. This table could also be used for failed attempts by thieves to read scrolls.
1. The spell-caster takes Xd4 damage, where X is the level of the spell.
2. The spell-caster falls unconscious for 10 minutes times the level of the spell.
3. If the chosen spell is one that affects creatures, every PC and NPC in the area, including the caster, rolls 1d6. Any that roll a 1 are affected by this spell.
If the chosen spell is one that affects an area, it is cast on an area centered on the caster, unless the caster intended it to be. In that case it is cast on an area centered on a randomly chosen character in the vicinity other than the caster.
4. As for 3 above, but randomly choose a spell of the same level. 50/50 cleric/magic-user. If reversible, 50% chance of being reversed. The result can be the same as the original spell.
5. The spell's effect is reversed. If the spell is not reversible, roll again.
6. The caster's movement rate is halved for as many hours as the level of the spell.
7. The caster is unable to benefit from magic healing, and their natural healing is halved (1 HP per 2 days), for as many days as the level of the spell.
8. The entire party takes X damage, where X is the level of the spell.