When a player casts a spell, they describe the effects they would like it to have. Nothing is impossible. As a spell is described, the GM will produce a DC for it, and decide which discipline(s) it falls under. Some effects may be achieved by a number of different disciplines - Flight, for example, can be achieved in numerous ways.
When the DC is exceeded by five points or more, the spell costs no Capacity. If you exceed by fewer than five, the spell costs the difference in Capacity. For example, if the DC for a spell is 25 and you roll a 30, the spell costs nothing, whereas if you roll a 27, it costs three Capacity. If you succeed in casting a spell but do not have sufficient capacity to cast it, the cost is deducted as 4 times the difference in HP (unless otherwise mentioned).
Spells that fail do nothing (except occasionally for Sorcerers). No matter what, the Capacity is spent. For example, a spell with a DC of 25 failed with a roll of 22 costs 8 capacity.
You can never spend more than 10 capacity on a single spell roll (meaning you can spend more on a mixed spell). If you roll a natural one (which still counts as a roll of 1) you can spend up to 15 capacity.
Determining DC, etc
See: Developing Spells
Spells usually cost 4 AP to cast, but there may be exceptions. Mixed spells or particularly powerful spells may cost 6 (or potentially more), though not always.
While most effects can be cast using only one discipline of magic, in some cases desired effects may only be achieved by combining two or more disciplines into a single spell.
The one big rule of making mixed spells is that you cannot use them to combine two discrete spells, and thereby reduce casting time. You cannot get away with casting a single spell that gives you flight and lets you shoot a fireball, but you can make a spell where you fly by shooting fire out of your feet.
To cast a Mixed Spell, the GM determines a DC for each discipline involved. This may be the same DC but often will not be. The caster then makes a check for each DC, succeeding or failing and spending Capacity as per the rules listed above.
If all checks are successful, the spell is cast. If all of them fail, the spell fails as above.
If only part of the spell fails, there is a 50% chance that the successful check(s) will carry through, often with unintended results. For example, perhaps one is casting a Flesh/Earth spell to make one's skin into stone. If the Flesh check is failed and the Earth check is successful, the caster's skin may turn to stone without having the flexibility granted by the flesh portion, and so they may be immobilized.
Spells whose effects last longer than a single round must be sustained by setting aside some Tenacity specifically for the spell equal to the amount of Capacity the caster spent in casting the spell.
- If a caster is using his tenacity to sustain spells and the tenacity cost of a new spell costs more than his free tenacity, sustained spells get dropped. The caster gets to choose which spell is dropped.
- The minimum Tenacity cost of a sustained spell is 1. If a spell's Capacity cost is 0, then its sustain cost is still 1.