First of all, my system is strongly based in the Advanced Dungeons & Dragons beliefs except that I see magick as a system of spell fatigue (i.e. Spell Points). The more learned you are (i.e. the higher your level) the more spell energy you are capable of manipulating. The system is no different for mages or priests. Mages draw upon the energies through skill & practice, while priests draw upon the power through worship and devotion (i.e. prayers).
Neither mages or priests must select the exact spells they will cast. Instead, they have a threshold of fatigue (i.e. 0 SP), which after it is surpassed they can no longer cast spells. Both classes can never cast spells of a higher level than is expressed in the Player's Handbook for their level. A mage simply wills to cast the spell of his choice, makes an Intelligence roll (See below), and the spell goes off (if he had sufficient SPs and the INT check was made). A priest simply pray for the spell they wish for their deity to grant them, make a Wisdom roll (See below), and the spell goes off (if he had sufficient SPs, the WIS check was made, AND they are still in favor with their deity). As you can see, this is an immediate way of determining if you have fallen from grace with your chosen deity.
The handling of reduction to 0 SP is up to the DM. I currently do not penalize the PC for this, but a rougher campaign my dictate that the PC is so mentally fatigued that he can no longer run, his movement is simply reduced, lapses into unconsciousness, etc.
I believe that no spell will go off perfectly on a routine basis. To this end, I have decided to go to the Spell Point system. The Spell Points are determined as follows for both mages and priests:
Each spell castable by the player is translated to Spell
Points
1st Level Spells: 1
2nd Level Spells: 2
3rd Level Spells: 3
4th Level Spells: 4
5th Level Spells: 5
6th Level Spells: 6
7th Level Spells: 7
8th Level Spells: 8 (mage only)
9th Level Spells: 9 (mage only)
For Example, a 1st level mage will have 1SP, he will have 1+1 SP if he is a specialist and the extra spell points can only be used toward specialist spells. A 5th level mage will have 11 SP (4 1st (4 SP) + 2 2nd (4 SP) + 1 3rd (3 SP) = 11 SP) and an additional 6 SP (1 1st (1 SP) + 1 2nd (2 SP) + 1 3rd (3 SP) = 6 SP) if he is a specialist. Priests gain extra SP from high Wisdom as normal and these SP may be used as normal SP for them.
The caster may determine how many spells of what level he/she will cast. The 5th level caster above could cast his SP allotment in all 1st level spells, all 3rd level spells (and one second), or any other combination. This makes for a much more versatile spell caster. No longer does a mage have to stand idly by because he memorized the wrong spell, but there is a down side. Whenever the mage is in a stressful situation (as decided by the DM), such as combat, or attempting a "short-cut", he must make a spell check. This roll is determined by the caster's Intelligence (for mages) or Wisdom (for priests). The die result must be equal to or lower than the caster's appropriate ability score and the higher the result (i.e nearer the mage's power score), the better the spell was cast.
For example, a mage of 17 Intelligence rolls a 17 on the die. This would be a textbook perfect spell (with exceptional effects if the DM desires), but were the roll an 18 then the spell would fail. A perfect 20 fails regardless of any modifiers and a 1 is always successful (though minimal).
It must also be noted that if a mage is casting a spell with no distractions (such as a read magic from within his own study), the spell roll is not required and the spell succeeds as normal.
The base roll is INT/0 for mages and WIS/0 for priests with the following
modifications:
For example, a mage with an 18 Intelligence is casting a fireball spell. The spell will cost him 3 SP. He rolls a bad initiative and wants the spell to go off quicker than the 3 segments required. He opts to cast the spell at a 1, so he takes a -2 to his roll, thus needing a 16. He could negate this penalty by dumping an additional 2SP into the spell, but he feels he may need the SP later. If the same caster were bound, gagged, and couldn't get to his components he could still try with a total -17 penalty (-5 for no verbal, -5 for no somantic, -5 for no material, and -2 for increased speed = -17). A roll of 1 would be successful. If the total penalties reduce the roll below a 1, then the spell cannot be attempted.
On a failed spell roll, the caster loses half (rounded up) his SP put toward the spell if the spell roll was missed by 3 or less. A missed spell roll by more than 3 or a perfect 20 results in all SPs put toward the failed spell.
When the mage finally rest, the mage studies his entire spellbook as a refresher. If spells have been removed from his books he cannot refresh his memory and thus cannot cast those spells.