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During the final battle with improved Irenicus, the only way I have been able to kill him is with incendiary cloud - as II seems to have high enough MR to withstand any other magical attack, this seems to show that incendiary cloud certainly does bypass magical resistance.

As a side note - a non-cheesy way of killing improved Irenicus with incendiary cloud is to load up a spell trigger with webs and blast them at him after hitting him with greater malison. Then toss two incendiary clouds on him, and he is toast.

The fact that IC bypasses II's magic resistance in my game may have something to do with difficulty setting - I have it set on hard not insane.

using Near Infinity, I tried to find anything about Incendiary Cloud that was different from Abi Dalzhim's and other spells that might explain it's bypassing magic resistance - the only thing of note, was that the spell category for most offensive spells is "offensivedamage" and for incendiary cloud, the spell catergory is "battleground."

I do not know the signficance of this.

I'm merely speculating, but this catagory difference is probably the reason.

Improved Jonnie the First's best defense vs your magical efforts isn't his Magic Resistance (which is 40%, by the way) - it's a specific clock he's wearing, sewed from the very fibers of 100% Swiss Cheese. :D
The Cloak confers immunity to various kinds of spell catagories.
Now, this is the "merely speculating" part - my guess is that the Cloak does not confer immunity to "battleground" spells.
Would you mind checking with NI what other spells are listed as "battleground"? Then we can see if the cloak protects the wearer against them, too.

Just got back from the Cotton Bowl, where Texas won a great game over LSU - I will check Near Infinity for this now - though from my quick perusal last night I couldn't find any other spell whose spell category was "battleground."

I did check sunfire, and it is categorized as "offensivedamage.

I'll update in a moment.

Could it be that IC deals out only elemental damage and no magical damage?

The following spells are "battleground" category spells:

Cloud Kill
Creeping Doom
Death Fog
Earthquake
False Dawn
Glitterdust
Grease
Incendiary Cloud
Insect Plague/Summoning
Stinking Cloud
Sphere of Chaos
Teleport Field
Time Stop
Web
Wish-Time Stop

For the most part, these spells do seem to ignore magical resistance.

Since these are the only "battleground" spells I could find, it stands to reason that the spell category "battleground" bypasses magic resistance.

Erm... From this list, only Earthquake, False Dawn, Teleport Field, Time Stop and Wish-Time Stop ignore magic resistance (IC is controversial). ;) The rest are stoped by MR.

I would not be surprised if it differs depending upon the patch-state of a game, but with my game's state, incendiary Cloud, Grease and Web definetly bypass MR - this is how I am killing the first incarnation of improved Irenicus - immobilizing area of effect plus icendiary cloud.

At least one of the three insect spells seem to bypass MR as well since the Fallen Planetar can hit improved Irenicus with insects.

I do not now about Glitterdust or Sphere of Chaos - I'll try 'em out next.

I will double check cloud kill and death fog as well - though I am pretty sure I have damaged the third incarnation of irenicus with at least one of these, using the same web-cloud tactic - though I did cast two incendiary clouds a death fog and a cloud kill on him at once so I might have misread the scrolling text.

Web bypasses MR in your game?! Wow, that unbalancing. Are you sure?
Maybe you used some Pierce Shield spells on Irenicus? His MR is merely 40, so a single Pierce Shield would actually make it -1% (assuming you're playing with the XP cap remover).

Again, maybe you lowered Jonnie's MR? Because the Insect spells are always blocked by MR in my game.

Same case - you probably lowered his MR, since i never heard of Cloudkill bypassing MR in a fully patched game.

I'll doublecheck - but I am fairly certain that web holds Irenicus without reducing his magic resistance - and Incendiary Cloud damages him. It has worked several times and hitting Irenicus with lower magic resitance and/or pierce magic has not been part of my battle process in the first round.

Though now that I now he only has 40% magic resistance, that one pierce magic I generally have left by the third round wil be of much more use!