Spell Casting System

Introduction
This system was originally created by Lord Blackfang on the Wizards boards in 2006. I updated it and modified it for better playability. This is meant to completely replace the Spell Points variant found in Unearthed Arcana, which is a broken piece of trash.

Many attempts have been made to produce a good point-based casting system, and every one of them has failed miserably, all because of the same basic design flaw: a linear - or even exponential - increase of casting cost for higher-level spells. It may seem like common sense because everybody's doing it, but in this case, everybody's wrong.

Increasing spell cost with level required casters to have absurd amounts of spell points at high levels, which in turn allows them to cast their most powerful spells way too often at the expense of lower-level spells. This can be fixed partially by charging casters for augmentation (like in the Expanded Psionics Handbook), but it requires an unreasonable amount of work and makes the system incompatible with additional material.

So what is the solution, you ask? Quite simple, really. Instead of making casting cost go up with spell level, make it go down with caster level. Yeah, that's the entire system in a nutshell.

Terminology
Mana: You have a pool of this. Casting spells drains from the pool.

Strain Tolerance: This is mana by a different name. Instead of counting mana down, casters accumulate strain points, and have a limit on how many they can accumulate. This is a purely cosmetic change -- people, generally, have an easier and faster time using addition (counting strain up from 0 to an upper bound) than subtraction (counting mana down to 0 from a pool) -- this can easily be inverted with no mechanical change.

Mana/Strain Cost: This table shows us the mana or strain cost (the costs are identical) of each spell. As you can see, the cost increases slightly with spell level (duh) but decreases with caster level - this is the most important feature of the system, the concept that finally made mana-based casting a real and balanced option.

The Rules
From here on out, the rules are described in terms of strain instead of mana, due to addition being a faster mental process than subtraction. All rules that are not specified (such as the difficulty classes of spells) are the same as in the core rules.

Strain Tolerance
Your strain tolerance is equal to your primary casting stat (the actual stat, not the stat modifier). For classes that have more than one casting stat (such as the Favored Soul), your strain tolerance is equal to the stat that would normally grant you bonus spells per day. You also gain additional Strain Tolerance equal to ½ your level in spellcasting classes and prestige classes that add to your caster level.

Casting a Spell
When a mana-based spellcaster casts a spell, she has to channel a portion of the mana she is using through her own body. This is taxing both physically and mentally, and is the basic limiting factor that determines how many spells a mage can cast without resting. Every spell has a Strain Cost, which depends on the level of the spell and the level of the spellcaster, as shown on the tables below. At high class levels, some spells have no Strain Cost, and this is fine - a powerful spellcaster can cast basic spells all day long.

Whenever a spellcaster casts a spell, she suffers Strain equal to the spell's Strain Cost. As she casts more spells, the Strain accumulates. As long as the total Strain a spellcaster has accumulated is lower than her Strain Tolerance, a spellcaster suffers no ill effect. Continuing to cast spells once her Strain is over her Tolerance, however, is extremely taxing on a spellcaster's body and mind.

As soon as a spellcaster's total Strain exceeds her Tolerance, she becomes fatigued (even if she is normally immune to this condition; this is fatigue of the mind, not the body). If a fatigued spellcaster wishes to cast another spell, she must first make a Fortitude save with a DC equal to (20 + the spell's level + the amount of Strain she has over her Tolerance). If the save is successful, the spellcaster casts the spell as normal. If the save is failed, the spell fizzles with no effect and the spellcaster becomes exhausted (even if she is normally immune to this condition). An exhausted spellcaster may not cast any more spells. A spellcaster ceases to be exhausted after an hour of complete rest. (Note: The DC on the Fortitude save can and should be changed based on specific classes -- I'd imagine Sorcerers would have lower saves, for example.)

If the spell is cast by a class that would normally ready spells every day, the "readied spell" is not forgotten and can be cast again, as long as the spellcaster has the available Strain to do so. The advantage that mana-based Sorcerers should have over mana-based Wizards is amount of Strain available.

Recovering Strain
You recover one-quarter your total strain every hour, but never more than your casting stat modifier.

Example: The Mage (formerly Wizard)
The Mage is what we're now calling the Wizard under this variant. They ready spells each day in the same fashion as their Wizard cousins, but cast from Strain/Mana instead. As noted under Casting a Spell, they do not lose prepared spells once they cast them -- instead, their spells are limited by their available Strain.

Changing Recovery Mechanics
This system has been designed so that the recovery mechanic can very easily be changed to suit your needs of your game. As written here, the system covers fatigue-based casting, recharge magic and, finally, a full recovery on rest for those who still believe in the 4-encounter work day. I expect most people to use the system exactly as written, but it has been engineered to be easy to add and remove these or other components to provide the recovery mechanic you want for your own game.

For an example let's say you don't like the recharge component as written, which allows you to recharge more strain and cast more of your higher-level spells as your caster level goes up. I believe this to be a good feature, but some will dislike the power curve it introduces to mages. To change this to better suit your needs, you can remove the current recharge mechanic and replace it with the following: You recover one-quarter your total strain every hour (but never more than your casting stat modifier). This divorces caster level from recovery and thus guarantees that a high-level spellcaster can't cast more of his top-level spells in a day than a low-level spellcaster can.