Saturday, July 28, 2012

[Greyhawk] Magic Resistance

Spirit of Greyhawk has been proceeding slower than I’d like, though it is still occurring! Now that most of the larger blocks of mechanics have been translated, the challenge has been to keep the Fate’s design fractal intact as I get the point of “filling in the blanks”. In other words, I’ve been trying to avoid creating new mechanics in SoG, even if the source material did.

My most recent instance of this fractal challenge has been Magic Resistance and consistency with Armor.

In the source material, Magic Resistance and Armor effectively do the same thing--make the possessor harder to hit. The distinctions between them are what's being used for the attack and the specific mechanic for that determination. So the attacker has to roll to hit then if the hit is successful, only then do you take damage into account. Neither provides a reduction in damage--only a reduction in the chance of taking damage.

SoG already reflects that Armor reduces the number of shifts of damage done, which seemed important as degree of success and damage are linked to the same roll in the Fate mechanic. While inconsistent with SoG’s source material, it has playtested pretty well. Stated differently, I don’t like the source material’s mechanic for “Armor Class” enough to translate it into Fate as is.

My initial take on Magic Resistance was more consistent with the source material: an additional chance that a magic effect has no impact. Initially I felt it was necessary to bring this assumption over intact because in the world of Greyhawk, the great consensus is that a 20% resistance has as much chance to resist a 1st level spell as it does a 3rd level spell. It turns out that this is not an entirely accurate assumption--in order to play the game as it was written, Magic Resistance is adjusted up or down depending upon the level of the spellcaster (DMG, p. 228)

Reviewing that definition for Magic Resistance gave me the following assumptions about how it acts in the gameworld:

  - Magic Resistance is separate from Saving Throw (i.e., opposed skill roll).
  - Magic Resistance can negate a magic effect BEFORE any opposed skill roll is rolled.
  - More powerful magic users can reduce or entirely offset the value of the magic resistance.
  - Magic Resistance is more effective against less powerful magic users.

So for me there was a decision point in translation: Accuracy or Consistency?

Here’s two different ways it might be handled in Fate. One is more accurate to the source material and one is more consistent (fractal) with existing mechanics.

“More Accurate” Magic Resistance

Translating the Magic Resistance Value

The source material is expressed in the source material as a percentage.  The defender must then roll that value or less.  To turn that into a 4dF difficulty that must be met or exceeded (as per the normal SotC mechanic), you could do the following:
10% Magic Resistance is +4
20% Magic Resistance is +3
30% Magic Resistance is +2
40% Magic Resistance is +1
50% Magic Resistance is 0
60% Magic Resistance is –1
70% Magic Resistance is –2
80% Magic Resistance is –3
90% Magic Resistance is –4
100% Magic Resistance is -5
This means that in its most basic form, something with 80% Magic Resistance must roll -3 or better on 4dF in order to resist the magical effect.

Basic Rule

When a magic effect impacts something with Magic Resistance, the defender rolls the dice TWICE:
The first die roll is to determine if Magic Resistance cancels the magical effect.  If Magic Resistance does not work then the defender rolls again... 
The second die roll (if the defender’s Magic Resistance fails) is for the defender’s Opposed Skill Roll.
The defender rolls the 4dF and if the roll MEETS or EXCEEDS the difficulty the defender is immune to the magic effect.

You could accomplish a similar effect by only rolling dice a single time and then translating that roll in those two contexts, but it didn’t seem to have the right “feel”.

Adjusting Magic Resistance

The skill of the caster has an effect on a defender’s Magic Resistance.  To be consistent with the source material (again, DMG, p.228) and some of the other general assumptions that SoG makes about levels of skill/power, adjust the Magic Resistance (as above) by the following rule:
Subtract 5 from the power of the Magical Effect and apply the difference to the Magic Resistance’s "difficulty".
Examples:
Spell Power +3 - (5) = -2 to the difficulty (makes it less difficult to resist)
Spell Power +6 - (5) = +1 to the difficulty (makes it more difficult to resist)
So putting it all together you’d have the following scenario:
Defender has a “base” 10% Magic Resistance, which means rolling +4 (or better) on 4dF.
Defender is targeted by a spell with a power of +4, which then adjusts the defender’s Magic Resistance to be:
+4 (Spell Power) - 5 = -1 modifier to the Magic Resistance of +4 = +3 or better must be rolled.
Defender rolls +2 on 4dF, so Magic Resistance doesn’t work and then the Defender rolls his Opposed Skill Roll (as per normal).

The Bottom Line

Though this feels like a fairly accurate translation, I don’t think it’s playable “enough”.

“More Consistent” Magic Resistance

Going to back a previous statement, there’s the gameworld assumption that Magic Resistance is similar to mundane Armor.  So this translation considers Magic Resistance consistent with Armor, just used against magical effects instead of mundane attacks.

Translating Magic Resistance

A closer translation of Fate Shifts to a 100% scale, means that each shift is worth between 15% and 20%.  For ease of translation, let’s stick with using 20% = +1 shift.
+1 Magic Resistance equates to Magic Resistance 1% to 20%.
+2 Magic Resistance equates to Magic Resistance 21% to 40%.
+3 Magic Resistance equates to Magic Resistance 41% to 60%.
+4 Magic Resistance equates to Magic Resistance 61% to 80%.
+5 Magic Resistance equates to Magic Resistance 81% to 100%.
By virtue of Fate's different level of granularity, there’s a bit of a fudge factor there especially in the 81% and up range, but I think it’s balanced out during play (see below).  So the reduction in shifts of effect is not entirely accurate with source material, it is more consistent with the changes that were made to mundane armor.

Basic Rule

Magic Resistance applies the shifts in the same way as armor protects from mundane attacks.
Example:
A creature with Magic Resistance listed in the source material as 40% (worth +2 in SoG) is attacked by a spell generated by a Wizard with a Magic skill of +4.
The Wizard casts the spell, the player rolls 4dF and gets +1 for a total of:  Skill (Magic +4) + Dice (4dF, +1) = Magical Power of +5
The creature has an opposing skill (let's say Willpower) of +2 and rolls +2 on 4dF.  Including the Magic Resistance, the opposed result is:   Skill (Willpower +2) + Dice (4dF, +2) + Magic Resistance (+2) = +6
The creature's +6 is greater than the Magical Power of +5 and the spell has no effect on the creature.

The Bottom Line

Though this translation allows for a reduction of magic effect (which doesn't exist in the source material), it is still consistent with what I think is the more important gameworld assumptions, is consistent with mundane armor and avoids the creation of another modified mechanic.
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