Tried this on a few mobs and found that with this talent specced, the damage done by a Firebolt on a Zombie is less than without it specced. I'm wondering if it reduces the resistance number by 40% whether it is positive or negative.
From what I can tell, the relevant resistance number modified by Power Caster is the one relevant to the spell cast - so, fire resist for Firebolt. Whilst this works more or less predictably for creatures with small resist (like Stags or Direwolves), the numbers are not what I would expect for creatures with higher resist. So, for example, a Firebolt cast on a Bear by a mage with 20 INT does 105 damage without Power Caster and 130 with Power Caster. If the Bear's fire resist (47%) were reduced by 40% (to 28.2%) you would expect more like 143. So maybe something else is going on when determining the reduction.
Power Caster only affects Magic Resistance (Magic Missile)
Firebolt is Fire Resistance
@Souly Yes, as Souly stated, spells in Fractured that have an elemental type do that elemental damage, not magic damage, and thus are unaffected by Magic Resistance, and the reduction of Magic Resistance doesn't come off the individual elemental resistances.
Yes, the tool tip for Power Caster refers to magic resistance, but this is not what I am seeing. Here are some damage numbers for different creatures using Firebolt and a 20 INT mage. The first number is without Power Caster. The second is with it. There are no other differences between the two cases.
Direwolf 175, 184
Rabbit 200, 200
Fox 200, 200
Bear 105, 130
Deer 200, 200
Stag 191, 194
Zombie 251, 234
You can see that where a creature has fire resistance, Power Caster has an effect. The numbers are roughly what you would expect if the fire resist were reduced to 60% of its starting value - with the exception of the Bear (which produces a result that doesn't seem to match the 60% number). The Zombie is a stand out as it is vulnerable to fire - ie. it has no fire resist. However, rather than making the Zombie more vulnerable to fire, Power Caster reduces the vulnerability of the Zombie to 60% of its starting value - with the result the performance of Firebolt is worse with the talent. This seems like a bug to me.
If the tool tip is correct, then all the above numbers should be the same, shouldn't they? In any case, the application of Power Caster to negative resist needs to be considered. I'll have a look at it with Magic Missiles.
I would also recommend a change to wording then. The Character sheet shows "Magic Resistances" and then a subtype in that category is "Magic Resistance".
Power Caster verbiage can be interpreted as being All resistances that fall under Magic Resistances or read literally as the only magical sub-type "Magic Resistance". Below would be some recommended changes I feel would be adequate with the thought that this talent is to affect only a single sub-type of magical damage. Ideally, the other 2 talents in the intelligence tree affect all spells, it is a little odd that it only affects a single sub-type that has only one spell in it.
Spells like Magic Missiles are given a different damage type name outside of magic. A common default would be "Arcane". Then adjust the tooltips appropriately - that leaves the "Magic Resistance" table on the character sheet untouched.
list itemAdjust Power Caster's tooltip: "Spells dealing Magic damage ignore 40% of the target's resistance." Right now only Magic Missiles exist I believe that do Magic damage, so if that is intended for end-game. Be specific and state "Magic Missiles ignore 40% of the target's resistance".
Re-label the character sheet from categorizing all resistances as "Magic Resistances" and just label it "Resistances"
Yes - agree.
Also, based on more Firebolts than I care to think of, I am pretty sure this talent is still treating resist as an absolute number - ie. regardless of whether the resist number is positive or negative, it reduces it by 40% - with the result that your best spells for any particular creature (because the creature is vulnerable to that spell) are nerfed by the talent.