What spell effects stack?

Alonso

It seems like some spell effects stack while others don't, but sometimes it's not clear what happens. For instance, let's say my wizard casts Protection from Evil and Blur on herself, and my Cleric casts Bless and Chant on the whole party. Then I fight an evil foe.

My wizard gets:
Saving throws: +2 from PfE, +1 from Blur, +1 from Chant.
Attack rolls: +1 from Bless, +1 from Chant.

The evil foe gets -2 to attack rolls from PfE and -1 from Chant.

If all of this stacks, she gets +4 to saving throws and +2 to attack rolls. Otherwise, it might be +2/+1, or something in between. Same for the evil foe.

So what is it?

Gallowglass

It's true that some effects stack and some don't, but there are so many effects that it's impracticable to list them (and anyway I'm not sure in every case).

In the particular case you mention, PfE + Chant + Bless + Blur ... yes, all of those stack. Getting well buffed-up for a major fight can make a huge difference!

Gotural

Generally speaking, every buffs stack except for spells like Armor and Shield because they both set AC to a particular value instead of increasing it. By the same logic you cannot stack Strenght and Strenght of One because they both set Str to a particular value, but you can stack Draw Upon Holy Might and Holy Power to easily reach 25 Str.

Something I'm doing for every major fight is Prot from Evil 10 Radius + Defensive Harmony + Mass Invisibility which increase everyone AC in the party by -8!

semiticgoddess

Pre-EE, many spells stacked with themselves. With EE, almost no spells stack with themselves, but they still stack with each other.

elminster

Gotural said:

By the same logic you cannot stack Strenght and Strenght of One because they both set Str to a particular value

Actually Strength and Strength of One should stack, since Strength increases your strength to 18/00 if your strength is between 18/50 and 18/00 (it gives an incremental bonus to your exceptional strength).

Alonso

Thanks for the explanations. So it looks like the general rule is: spells don't stack with themselves but they do stack with other spells.

Is there any (non obvious) exception to the rule that you can think of?

semiticgoddess

@Alonso: Boon of Lathander and Called Shot stack with themselves. In IWD, the Lucky Scimitar and the Luck spell do not stack, as they use a non-cumulative opcode for luck. Also, Bhaalspawn innates like DUHM and Vampiric Touch don't stack with their cleric and mage equivalents.

Holy Power and Champion's Strength partially stack. The Strength bonuses don't stack, since they set the target's STR to 18/00, and Holy Power does not improve the caster's THAC0 with multiple castings, but Holy Power does give more HP with extra castings, and Champion's Strength further improves THAC0 if cast multiple times.

An Arrow of Dispelling from a clone can let you remove the negative side effect of Champion's Strength, which is to disable the caster's spellcasting.

Flashburn

These don't stack, per-se, but Haste doubles your regeneration rate (and poison damage rate!)

Also Assassin/Blackguard poison apply their effects to all hit by an Arrow of Detonation.