Combining Magic Effects
Spells or magical effects usually work as their descriptions state, 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.
Spells and effects that provide bonuses or penalties to attributes such as attack rolls, damage rolls, and saving throws usually do not stack with themselves if multiple effects would apply to the same attribute. More generally, two bonuses of the same type do not stack even if they come from different spells or from effects other than spells (see Bonuses on page 266).
However, damage from multiple spells that deal damage is always cumulative.
In cases when two or more spells produce identical effects in the same area or on the same target, but at different strengths (such as one spell granting fire resistance 5 and another granting fire resistance 10), only the one with the highest strength applies. If a previously cast spell lasts longer than a more recently cast spell producing the same effect, and the most recent version expires, the previously cast spell resumes its effect for the remainder of its duration.
Multiple Mental Control Effects
Sometimes magical effects that establish mental control render each other irrelevant, such as spells that remove the subject’s ability to act. For example, a creature under the effect of a hold person spell cannot be compelled to move using a dominate spell, because the hold person effect prevents the creature from moving.
Mental controls that don’t remove the target’s ability to act don’t usually interfere with each other. If a creature is under the mental 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 attempt opposed Charisma checks to determine which one the creature obeys.
Countering and Negating
Some spells can be used to counter other specific spells, as noted in their spell descriptions. For instance, you can use slow to counter a casting of haste. This works exactly like the counter effect of the dispel magic spell (see page 351), except you don’t need to attempt a caster level check; if the target is in range, the spell is automatically countered and fails.
Many times, these same spells note that they negate one another as well. This means that a successful casting of one spell on a target under the effects of the second spell undoes those effects, and the effects of the first spell don’t occur.