Experimental Spell System

Introduction
Ian mentioned that there aren't yet clear rules for spells in the game. As I like playing magic-users, and have some sort of twisted fascination with magic systems, I thought I would try my hand at generating some sort of magic system, so here are my ideas.

Concept
Create a casting system that prevents casters with a low magic skill from casting spells that seem clearly too powerful for them to cast, while letting those with higher skill benefit from being able to use higher powered spells. To this end, I think the simplest solution is to give a spell a "spell value" (SV), which can then be used to determine whether or not the caster is able to safely cast the spell. I say safely because I have also included "adverse effects"--that is miscasting--to make things more interesting, and more dangerous. This way those with lower skill may attempt to cast a high-powered spell, but also run the risk of hurting themselves or others unpredictably.

Spell Value
A spell's SV is a number, generally between 1 and 10 (but which may be higher), that describes how powerful a given spell is, and thus how difficult, and dangerous, it is to cast. A simple, low-powered touch damage spell would have a spell value of 1 (SV1) in my mind, whereas a spell like blizzard or meteor shower would perhaps be 10 (SV10). In the middle (SV5) might be a moderately powerful fireball that does some area damage. This system can also be made more flexible by allowing the caster to add or subract spell value from a given spell. For example:

A damaging touch spell is SV1 base. To me, this means it requires touch range and only rolls one damage die. If the caster wanted it to do more damage by rolling another damage die, that would make it SV2. Conversely, if the caster wanted to make it a ranged fire bolt spell that still only rolls one damage die, then it would also become SV2. To make that firebolt spell roll 2 damage dice, it coulither be SV3 or SV4 (depending on whether the cost of +1 damage die is higher for a ranged effect or not, or GM discretion).

Spell value is, in effect, a percentile. So SV5 means a spell that is 50% challenging, SV8 = 80%, SV10 = 100%. No combat-viable spell can ever be SV0, which is about the spell level of lighting a candle with magic.

Spell Value's effect on Spell Casting
The gist of the system is this: If the SV as a percentile is less than your Magic skill, then you may cast the spell without adverse effects, whether or not you actually succeed in casting the spell. If the SV is higher than your Magic skill, then you risk miscasting. To be more precise, when casting a spell with a SV greater than your skill, a negative modifier equal to your skill minus the SV percentile is applied when trying to cast the spell. If you roll under the modified Magic skill value, you cast the spell just fine. If you roll above the modified skill value, but below the base skill value, you manage to cast the spell but suffer adverse effects in doing so. If you roll above the base Magic skill value, you fail to cast the spell as normal, but still suffer miscast effects because the spell was too powerful for you to really control.

An Example
Malcador has a Destruction skill of 40. If he were to cast a simple firebolt (SV2), he could do so without risk because his Destruction skill (40) is greater than (or equal to) the spell value percentile (SV2 = 20). This would obviously be the case for spells up to SV4 (40). If he wanted to cast a powerful fireball spell (SV5), he would have to risk miscasting. There would be a modifier applied to his Desctruction skill equal to the difference between his Destruction skill value and the SV of the spell he wanted to cast. In this case, this modifier would be -10 (Desctruction skill 40 - SV5 (50) = -10), and his "safe casting range" would be reduced to 30. If he rolls under 30, the spell goes off without a hitch. A roll of 30-40 means that the spell is cast (at full strength), but Malcador must then roll on the miscast table. With a roll higher than 40 the spell fails, but he must still roll on the miscast table.

This also means that there may be spells that are completely unsafe if your magic skill is too low. For example, if Malcador wanted to cast a spell with SV8, he would be suffering a -40 modifier to his skill (Destruction 40 - SV8 (80) = -40). Of course if he rolled under a 40, the spell would still go off, but he would suffer miscast damage from even trying to cast the spell. If the negative modifier is ever larger than the base skill, then the difference between those two goes on to modify the base skill itself. That is, if Malcador tries to cast a spell with SV10, his modifier would be -60 (Destruction 40 - SV10 (100) = -60). This of couse means that his safe casting threshold is nil, but also modifies his base Destruction skill by -20 (-60 + Destruction 40 = -20) when trying to cast the spell. Therefore, rolling below 20 means the spell is cast and miscast effects applied (since his safe threshold is nil), while rolling above 20 (his now-modified base skill) means that the spell cast fails and miscast effects are still applied.

Raising one's magic skill therefore not only increases their ability to cast spells, but also reduces the risk associated with miscast. Those with a high skill, then, can cast spells that are one or two SV above their safe limit somewhat safely than those with lower skill. This is in line with the idea that a skilled sorceror should be able to use high-powered magic relatively safely, as compared to apprentices who often incinerate themselves while trying to outperform their skills.

Miscast
When one suffers adverse effects they must roll on the miscast table (unless they pass a certain check explained below). If they miscast by a large degree, they have to make more than one roll. The amount of rolls they have to make is equal to the tens place of the difference between their safe threshold and the actual roll. If your safe threshold was 30 and you rolled a 63, you have to make three rolls on the table (63-30 = 33 > tens place = 3). The results of these rolls are always cumulative, so if you roll damage three times, you roll three damage dice and apply the total. Here is the preliminary table based on rolling a d10:

0 ~ No effect. Lucky...

1 ~ Immolation: 1 Damage die on self

2 ~ Explosion: 1 Damage die to all within d10 feet (Including caster)

3 ~ Stray Bolt: 1 Damage die on random (or GM discretion) person or object in area

4 ~ Stray Blast: As above, but with Explosion effect on the target

5 ~ Magic Rip: Caster cannot cast any spells for the next combat round or 30 seconds (cumulative)

6 ~ Magic Rend: As above, but effect spreads to all within d10 feet.

7 ~ Random Magic: Caster accidentally casts another spell with the same SV as the one they attempted instead of or as well as the spell they tried to cast. The target of the spell is the same as the one they attempted.

8 ~ Chaotic Magic: As above, but with a random target.

9 ~ Huge Fuckup: GM Discretion (This may include summoning an uncontrolled monster, accidentally eating somone's soul, attracting the attention of dark gods, etc... And it may also be made worse if you roll more than one 9 on the table when trying to cast a single spell. I.e. if you miscast and have to make 3 rolls, all of which are 9s, then you might get a really pissed off Balrog or some such. Let the GM decide the level of hurt).

Rolling on this table is simple, make the rolls and immediately apply all the effects you rolled.

Controlled Miscast
One can imagine an intelligent or spiritually powerful caster would more likely screw up in the safest way possible, as they knew that what they were trying to cast was dangerous, or were just damn lucky. To represent this, right before casting a spell the caster may make a roll against their magic stat (intelligence or spirit, not the specific skill itself). If they pass, they may choose their miscast results rather than rolling them randomly (though they may never choose "No Effect"). When making this roll, the magic stat will have the same negative modifier as the Magic skill, that is, with a skill of 40 and a spell with SV7, you would be making this roll with a -30 modifier. If this roll is successful, you may choose the results (rather than rolling) of a number of miscast effects equal to the tens place of the roll. For example, you have a Magic skill of 40, and you are trying to cast a spell with SV6. This would mean a -20 modifier to your magic skill (or at least to your safe threshold). So if you wanted to control the miscasts because you are casting a dangerous spell, you would have to roll against your Intelligence (in the case of Arcane magic, Spirit if Divine). That roll would have the same modifier as the actual casting roll (-20 here). So say that your intelligence is 60, you would have to roll under 40 (60-20 = 40) to pass the roll. Say that you roll a 23, which means you can choose two miscast effects rather than rolling (tens place of 23 = 2). You then roll a 52 on the casting roll, which is greater than your skill of 40, and so the spell fails and you take three miscast effects. Because you rolled 23 against your magic stat to control the miscast, you may choose two effects on the table (or one twice), but must roll for the third.

However, there are limitations to trying to control your miscasts. By making this roll, it means that your character is taking special preparations prior to beginning to cast the spell, and if these preparations are stopped, so is the spell cast. Therefore, if you fail to control your miscasts, you also fail to cast the spell, but suffer no miscast effects. Also, you must make this roll before attempting to cast the spell at all. It is only a preventative measure to mitigate the damage caused by miscasting.

Anyway, that's my idea. I'd love to hear comments or questions about it.