The default rule for the interaction of any two esoteric systems is simple: the effects of one system (spells, powers, veils, etc.) interact with the effects of the other system and effects of the other system interact with effects of first system in the same way a spell or spell-like ability interacts with another spell or spell-like ability. This is known as system transparency.
For purposes of this section, all magical things (spells, powers, veils, auras, spell-like abilities, general magic effects, and so forth) will be referred to by the collective term “esoterica” or “esoteric effect.” You will be able to substitute spell, power, or any other such term in the place of “esoterica” when necessary to understand the rules.
Multiple Effects
Multiple esoterica usually work as described no matter how many other spells, powers, veils, or other magical effects happen to be operating in the same area or on the same recipient. Except in special cases, an esoteric effect does not affect the way another effect operates. Whenever an esoteric effect has a specific effect on other esoterica, the description explains the effect (and vice versa for esoterica that affect it). Several other general rules apply when multiple esoterica operate in the same place.
Stacking Effects
Esoterica that provide bonuses or penalties on attack rolls, damage rolls, saving throws, and other attributes usually do not stack with themselves. More generally, two bonuses of the same type don’t stack even if they come from different esoterica, or one effect from one system and one effect from another system. You use whichever bonus gives you the better result.
Different Bonus Types
The bonuses or penalties from two different esoteric effects stack if the effects are of different types. A bonus that isn’t named (i.e., a “+2 bonus” rather than a “+2 insight bonus”) stacks with any bonus.
Same Effect More than Once in Different Strengths
In cases when two or more similar or identical esoteric effects are operating in the same area or on the same target, but at different strengths, only the best one applies. If one effect is dispelled or its duration runs out, the other remains in effect (assuming its duration has not yet expired).
Same Effect with Differing Results
The same esoteric effect can sometimes produce varying effects if applied to the same recipient more than once. The last effect in a series trumps the others. None of the previous esoteric effects are actually removed or dispelled, but they become irrelevant while the final effect in the series lasts.
One Effect Makes Another Irrelevant
Sometimes, an esoteric effect can render another effect irrelevant. Simply ignore the irrelevant effect while it remains irrelevant.
Multiple Mental Control Effects
Sometimes esoterica that establish mental control render one another irrelevant. Mental controls that don’t remove the recipient’s ability to act usually do not interfere with one another, though one may modify another. If a creature is under the control of two or more creatures, it tends to obey each to the best of its ability, and to the extent of the control each effect allows. If the controlled creature receives conflicting orders simultaneously, the competing controllers must make opposed Charisma checks to determine which one the creature obeys.
Esoterica with Opposite Effects
Esoterica with opposite effects apply normally, with all bonuses, penalties, or changes accruing in the order that they apply. Some esoterica negate or counter each other. This is a special effect that is noted in an esoteric effect’s description.
Instantaneous Effects
Two or more esoterica with instantaneous durations work cumulatively when they affect the same object, place, or creature.