Casting a spell is relatively straightforward and requires a minimal amount of math. Note that various modifiers may apply to the casting check; these are optional but recommended.
Casting Check
A spellcasting check relies upon the character’s Spellcasting proficiency, as such:
Casting Check = d20 + Proficiency Bonus + INT / WIS
vs Casting DC
If the caster is wearing Medium or Heavy armor, ze makes hir casting check with Disadvantage.
If the caster has the Medium Armor Mastery feat, ze does not suffer Disadvantage while wearing medium armor.
If the spell’s level is above the caster’s combined proficiency level in the appropriate Arts, ze makes hir casting check with Disadvantage.
The caster applies the following optional modifiers to this check based on hir circumstances:
- If the caster speaks hir incantations in a loud voice that draws attention, ze may add a bonus of +1 to the check.
- If the caster makes exaggerated gestures that are easily spotted, ze may add a bonus of +1 to the check.
- If the caster must speak quietly during spellcasting to avoid notice, ze suffers -3 to the check. If ze cannot speak at all, the penalty increases to -5.
- If the caster must make subtle gestures while spellcasting to avoid notice, ze suffers -1 to the check. If ze cannot gesture at all, the penalty increases to -3.
- A mage can expend raw vis matched to the spell’s Technique or Form for a bonus to the check: +1 per pawn of vis. The maximum amount of vis a mage can use equals hir Magnitude in the matched Form or Technique.
- If a mage has mastered the spell ze is casting, ze adds hir mastery ability as a bonus to the casting check.
Casting Check Modifiers
What happens is based on whether the result beats the DC:
- If the unmodified roll for a spell cast under stress falls within the spell’s Botch Range, the spell botches and the caster gains Warping Points. See below.
- If the check is below the Casting DC by more than 5, the casting fails.
- If the check is below the Casting DC by 5 or less, the casting fails; however, the caster can choose to suffer a level of fatigue and have the spell go off successfully. If so, the Penetration Total will be negative. The spell will fail against any target with Magic Resistance.
- If the check is above or equal to the Casting DC, it generates a Penetration Total and invokes the formulaic or spontaneous spell or ritual successfully.
The Ability Bonus uses INT or WIS, whichever is higher.
Let’s assume Tisealee is casting her formulaic spell Pilum of Fire (20 CrIg), a Level 20 Creo Ignem attack spell that shoots fire at an enemy. Her total Spellcasting bonus is +5 and she only has 1 Botch Die for this casting. That means she only botches on a natural roll of 1. This is a Magnitude 4 formulaic spell, so its casting DC is 14. She rolls 21 for an excellent success.
Casting DC
The Casting DC for the check depends on the type of spell and the Arts required to cast it.
For a Formulaic Spell, the DC is:
Formulaic Spell DC = 10 + Spell Magnitude
For a Spontaneous Spell, the DC is:
Spontaneous Spell DC = 10 + (2 x Spell Magnitude)
The spell’s magnitude, of course, equals its (Form + Technique) / 5, rounded up.
This is the base casting DC; the DM can choose to apply bonuses or penalties, based on the circumstances of casting.
Botches
If the mage is casting a spell under casual conditions, it’s not usually subject to botches.
Any time the mage casts a ritual, or if ze casts a spell in a stressful situation (like combat,) the casting check has a potential to Botch.
Most casting checks botch if the casting check die comes up a natural 1. If the mage used vis to cast the spell, as in a ritual, increase the Botch Range by 1 for every 2 pawns of vis (round up). For a ritual that uses 4 pawns of vis, for example, the Botch Range is 1-3. For a formulaic spell on which the caster spends 1 pawn, the Botch Range is 1-2.
On a botch, the caster gains a number of Warping Points equal to the (unmodified) roll. The casting total is 0 and the spell goes off; however, its effect is not what the caster intended and is often detrimental. If a mage gains two or more Warping Points from a single botch, ze may enter Wizard’s Twilight. (See Ars Magica 5th Edition p. 87-88 for more information.)
Penetration Total
There is one other consideration when facing magical beasts or fellow spellcasters in combat: overcoming the target’s Magic Resistance.
Penetration Total = Result + Penetration Bonus - Casting DC
The Penetration Total for a successful spell equals the result minus the DC. If the caster has a Penetration Bonus, it’s added to the total. Note that this can be a negative number.
Most creatures and objects do not have Magic Resistance; for them, the spell simply succeeds or fails and takes its effect accordingly. For creatures that possess an innate resistance to magical effects, though, the spell’s Penetration Total is compared to the target’s Magic Resistance. If the spell’s Penetration Total is greater than the target’s Magic Resistance, the spell takes full effect on the target. If the Magic Resistance is greater, the casting succeeds but the spell completely fails to take effect on the target.
Tisealee made 21 on her casting total and she has a Penetration bonus of +1. Since the DC was 14, that makes her Penetration Total 8. Her target is an enemy spellcaster with Ignem 3 and an effective Parma Magica of 1, so her Magic Resistance is 4. Tisealee beat her foe’s MR. That means her spell takes full effect. The target can still make a Saving Throw vs. Spell for half damage, but the spell is going to hurt, one way or another. Burn, baby, burn!
When a caster uses an Arcane Connection, it provides a multiplier bonus to the spell’s Penetration Total based on the strength of the connection, making the spell much more likely to effect a target with high Magic Resistance. In addition, the mage can use Sympathetic Connections to increase the multiplier further.
Casting Time
A Formulaic Spell has a casting time of 1 action.
With Fast Casting, you can cast a Formulaic Spell with a Bonus Action or Reaction; however, this applies -5 to the casting check.
Normally, a Spontaneous Spell has a Casting Time of 2 Actions. (i.e., it goes off on the caster’s second turn after casting.) The spell is subject to interruption and requires a Wisdom save for concentration from the mage if anything distracts hir while casting.
With Fast Casting, you can cast a Spontaneous Spell in a single action; however, this applies -5 to the casting check.
Spell Resolution Notes
Spell Save DC
The DC to save against a magus’ spell is based on the caster’s Form level and INT or WIS modifier:
Spell Save DC = 10 + Caster’s Magnitude in the Form + Caster’s INT / WIS (whichever is higher)
Dealing Damage
When a spell deals damage, it does 1d6 per +5 damage in the effect.
Healing Damage
When a spell heals damage, it heals a percentage of the target’s total HP based on the wound level the spell heals.
Any healing spell can restore lost HP, but it takes a magic ritual to heal wounds and physical damage permanently. Otherwise, the healing effect only lasts for the duration of the spell.
Wounds Healed / Inflicted
Inflicting Wounds
When a spell inflicts a wound directly, it inflicts a percentage of the target’s total HP based on the wound level the spell inflicts. See Wounds Healed / Inflicted, above.
Modifiers
When a spell grants a bonus or imposes a penalty, the modifier is generally the same as that specified by the spell. If the DM rules the modifier needs adjusting, ze makes the final determination.
Recovery
When a spell grants a bonus to Recovery rolls, it adds to any hit dice the character spends to recover hit points during the next Short Rest. If the character takes a Long Rest first, ze regains all hit points and hit dice as usual but loses the bonus.