Spellcasters with this system are not limited in the number of spells a day they can cast. Damage and fatigue will determine this.
I envision something like this:
- In order to cast a spell without taking damage you must roll a d20 and beat the spells DC or suffer fatigue. D20 + appropriate stat modifier + ½ character level must be equal to or better than 12 + spell level.
A natural roll of 1, in addition to fatigue, indicates that the caster has suffered damage (mana burn) equal to d6 x the spell level (do not multiply the d6 roll, just roll a number of d6’s times the spell level.)
The first fatigue roll failed in an encounter by a caster causes him to suffer a -1 modifier on all rolls and loses 5ft of movement till the end of the encounter.
A second failed fatigue roll causes the caster to become exhausted. When exhausted a caster suffers a -3 modifier to all rolls and loses 15ft of movement till the end of the encounter.
A third failed fatigue roll in an encounter causes the caster to become unconscious and automatically take spell energy damage as if he had rolled a 1.
After the encounter is over the spellcaster partially recovers from the fatigue. They no longer suffer the negative modifiers to rolls and movement but any "mana burn" damage still remains.