Be sure to consult the core rules on spell logic when designing a new spell. A spell should be no more than a paragraph long. Here are some things to consider when designing the spell.
Checklist for New Spells
- Make sure a similar spell doesn't already exist.
- The spell should not do damage directly.
- Consider all the parameters of the spell (area of effect, duration, and targets) to see if it deviates from what's assumed by the global spell logic. If it does deviate in some way, outline that deviation in the spell's text.
- Determine if the spell affects a number of targets, an area of effect, the spellcaster only, or any target other than the spellcaster.
- Determine if the spell requires concentration. This means the spellcaster cannot cast other concentration spells while concentrating on this one. (We often add concentration as a requirement to balance a spell's power.) If the spell does not require concentration and has a duration, then it's a directed spell rather than instantaneous.
- If the spell has inversion, describe the spellcaster's options in the spell text. (For example, Haste or Slow can do one or the other whenever it is cast.)
- Does the spell have a wager? If so, describe the options at each amount of MP invested in the spell's text.
- If the spell summons something and you want the summon to have specific stats (as opposed to what is assumed by the global spell logic), describe those stats in the text of the spell.
- Consider what limitations should be imposed upon the spell, such as types of targets, whether it requires touch to cast, and whether it can be cast as a reaction.
When writing out the spell, you don't want to restate anything that is already assumed by the rules of spell logic. So for example, there's no need to say that a spell lasts for a number of rounds equal to the MP invested, because that's already the case for all spells. You only want to include verbiage that deviates from those assumptions—e.g., the spell lasts for a number of days per MP spent.