Can spells with cumulative or instantaneous ongoing effects affect a creature multiple times?

Question

In the current campaign, the levels 2 and 3 PCs want to spam foes with the 1st-level Sor/Wiz spell cause fear [necro] (PH 208) and the 1st-level Sor/Wiz spell power word pain (Races of the Dragon 116), the former so that they can corner and beat into unconsciousness the rare creature that they want to take alive and the latter so that they can perpetrate dungeonwide loot-their-corpses-after-they-die-in-4d4-rounds murder sprees.

It seems reasonable that a creature affected twice by the spell cause fear becomes frightened rather than merely staying shaken for a new, longer duration. Likewise, it seems reasonable that a creature affected by two or more power word pain spells would be in more pain—and suffer more damage—rather than the new power word pain spell's duration overwriting the previous's.

Nonetheless, since these aren't, like, polymorph effects or temporary hp or anything with a specific rule, I am consumed with self-doubt and suspicious of both tactics, thinking they conflict somehow with the rules about the Same Effect More than Once in Different Strengths (cf. Player's Handbook 172), the rules about the Same Effect with Differing Results (ibid.), or both… or another rule somewhere.

Can these kinds of spells—that have ongoing durations but either cumulative different effects or instantaneous effects—affect the same creature more than once simultaneously?

Answer 1

From Player's Handbook page 171:

COMBINING MAGICAL EFFECTS

This broad category begins with the primary rule of the section.

Spells or magical effects usually work as described, no matter how many other spells or magical effects happen to be operating in the same area or on the same recipient. Except in special cases, a spell does not affect the way another spell operates. Whenever a spell has a specific effect on other spells, the spell description explains that effect. Several other general rules apply when spells or magical effects operate in the same place:

So, as a baseline, everything stacks except for the exeptions it defines. The list of exceptions, in addition to special cases listed in the actual spell descriptions, includes these topics:

Stacking Effects: This section begins by explicitly defines what stacked effects it refers to:

Spells that provide bonuses or penalties on attack rolls, damage rolls, saving throws, and other attributes usually do not stack with themselves.

It then goes on to describe italicized sub-categories of "Stacking Effects" which all fall under the initial umbrella defined in the first sentence of the category section. I.e. "Same Effect More than Once in Different Strengths" is a sub-category of "Stacking Effects," so this sub-category applies only to spells affected by the "Stacking Effects" rules.

Spells with Opposite Effects: This section does not limit application of effects, but simply describes in what order to resolve them excepting for spells with built-in special-case interactions.

Spells with opposite effects apply normally, with all bonuses, penalties, or changes accruing in the order that they apply. Some spells negate or counter each other. This is a special effect that is noted in a spell’s description.

Instantaneous Effects: This section removes instantaneous effects from the rules set forth in "Stacking Effects"

Two or more spells with instantaneous durations work cumulatively when they affect the same target.

Unfortunately, the "Instantaneous Effects" section includes an example of a damage spell. This adds a lot of confusion, because without this example, there is no question about damage being limited or not.

For example, when two fireballs strike a same creature, the target must attempt a saving throw against each fireball and takes damage from each according to the saving throws’ results. If a creature receives two cure light wounds spells in a single round, both work normally.

Note that an example does not create rules. The rules have not explicitly defined damage as an exception to the primary rule set forth in the "COMBINING MAGICAL EFFECTS" section that clearly states things work as described no matter what except for the defined special cases.

ANSWER 1 of 2: Yes, power word: pain stacks because damage is not restricted from stacking per the PHB rules.

ANSWER 2 of 2: No, cause fear will not stack.

Explanation of the difference:

From the Dungeon Master's Guide page 294:

Becoming Even More Fearful: Fear effects are cumulative. A shaken character who is made shaken again becomes frightened, and a shaken character who is made frightened becomes panicked instead. A frightened character who is made shaken or frightened becomes panicked instead.

However, because spell interactions are defined in the Player's Handbook, and the Player's Handbook specifically states that the same spell cannot stack with itself, then you would need a new source of additional fear effect to stack with the fear effect from cause fear since the PHB overrules the DMG specifically for spell purposes. The RAW here would prevent any additional cause fear spells from generating any effect at all.

Answer 2

A second application of power word: pain is a clear case of the same effect in differing strengths (namely, whatever you roll). At best you could argue that you would roll 1d6 twice each round and take the better roll for the damage of that round, though personally I wouldn’t even do that.

I also would not allow repeated applications of the same fear effect to escalate fear. This is not clear from the rules (you could make an argument about fear causing penalties on various rolls and so is covered by “Spells that provide bonuses or penalties on attack rolls, damage rolls, saving throws, and other attributes usually do not stack with themselves,” but it’s a bit of a stretch and then you have to argue whether that’s more specific than the fear-escalation rules), but it is my understanding of a general consensus on how things work.

Answer 3

with a duration effect, the better of the two effects takes place, although if one of the effects is canceled, the other still remains. This is true of effects like fear and charm.

With instant effects that create a change, the instant effects, like being struck by level draining or fire damage, stack.

Conditions of the same category do not stack unless the penalties or bonuses are of a different origin. For example, an insight bonus/penalty stacks with a natural bonus/penalty, but two natural bonuses/penalties do not stack, likewise, pain spells and effects do not overlap, but any damage, such as from poison, may stack.

As a rule of thumb, same flavor penalties don't stack, but damage does. Some DMs may rule that same flavor AOE damage in the same initiative does not stack, such as two overlapping fireballs. This saturation effect is probably what you are looking for.