Mana storms blow up with appropriate pyrotechnic effects in magically charged wilderness areas, where they will be big or can arise as a terrible side effect of spell fumbles, probably in a smaller area.
Some possible effects, which might shift over the duration of the storm
The storm should have special effects and add to the special effects of magic used within. In addition, it can have a variety of possible concrete effects.
- Heat metal as per druid spell on magic armor & weapons & metal items.
- Restore spells to spellcasters (maybe only arcane casters?)
- Make spells stick in memory - in a memorized spell system, mages save vs magic (or WILL save) or 1-6 spells get stuck in memory and cannot be forgotten until they are fumbled on casting or enough time passes. They still get scrambled by casting half the time so the mage doesn't get too much extra casting out of them, but they cannot be pried out of the spell slot to be replaced with another spell. In a spell points system, they hold onto the points they have committed to them. In DCC, they won't be forgotten on a miss except a fumble, but will misfire(1-3) or pick up an extra mercurial magic side effect. After each day's sleep roll 2D6 for each stuck spell, and on a 12 it loses its grip and can be normally replaced. A Forget spell can clear a stuck spell immediately, but a crit result will make the mage actually forget how to cast it entirely until relearned (twice as fast as originally learning it) or he levels up.
- Clear 1D6 spells from memory - an inverse to stuck spells, 1D6 spell slots are wiped clean. For each, roll again and on a six the spell is truly forgotten and cannot be cast until relearned or the caster levels up. If a spell is forgetten in a spell slots or spell points system, the slots or points can be used to prepare a different spell the next day.
- Each caster in the storm makes a luck roll. If lucky, spellcasting will be at a +1D6 bonus to the roll for the duration of the storm. If unlucky, a 1D6 penalty. Roll bonus or penalty at each spellcasting. If not using a casting roll, then apply an extra die to damage if appropriate or adjust saves versus the spell by half the bonus, whichever direction is appropriate for caster's luck or ill luck.
- Variable effects to spells. , 1 - add 50% to damage/intensity, 2 reduce it by half of an appropriate axis of power, 3 - 12 come up with a weird side effect (extra mercurial effect if DCC).
- Dramatic increase or decrease in duration, even inversion of permanent vs temporary.
- Intensify up to double the powers of magic items in the storm, and add a wild magic/mercurial magic effect. Roll 1d6 at end of storm: 6 the change is permanent.
- Weaken magic item effects in the storm.
- Warp targeting of spells and ranged items in the storm. 1- overshoot target by half of range & hit nearest character to that point, 2- undershoot by 1/3 & home on nearest, 3 - double area of effect, 4 - half area of effect, 5-6 roll 1D6 and count away that many potential targets from the intended target, and retarget.
Some magical gems might change color in response to being in an area prone to mana storms or when a storm is imminent.
Storm sense might be a skill or power of mages or magical creatures.
The duration of the storm should be whatever is most interesting dramatically.
Plants or sessile creatures endemic to the storm zone might be charged up with potion effects if harvested in or after the storm and consumed.
There's a list at the bottom that I'm not sure what I intended:
- Red Flux - Bloodthirst "War flux"
- Black Flux - Death and Undeath, "Death flux"
- Green flux - Peace, calm "Peace flux"
- Blue flux - Magic discharging "Lightning flux"
- Purple - Illusion, change, mutation "Chaos flux"
- Yellow - Reality, Reversion, Stasis "Law flux"
- Orange - Magic charging "Power flux"
- White - Healing & life "Life flux"