Spell Effect vs Spell Effect
Sometimes one spell interacts with another, and the rules are either vague about what happens, or the effect is black and white — the spell works or it doesn’t. So, unless a given spell has specific rules governing its interaction with other spells that include a die roll, the following rule applies:
Any time two spell effects come into direct opposition, the spells make opposed caster level checks with a bonus to the check equal to the spell’s level1. Any spell that is specifically written to explicitly or automatically overcome or resist against another spell gains +10 to this check. The effect of the one that wins dominates the interaction appropriately. Ties go the spell that initiated the conflict.
Example 1: Suppose a 3rd level wizard casts invisibility to sneak past a guard post, not knowing that one of the guards is a 5th level wizard who is under the effect of see invisibility. When the 3rd level wizard steps into view, the “Spell Effect vs. Spell Effect” rule kicks in (because normally see invisibility trumps regular invisibility without a roll). The two spells make opposed checks: 1d20+5 for invisibility (+3 for the caster’s level, +2 for the spell’s level); 1d20+17 for the see invisibility (+5 for the caster’s level, +2 for the spell’s level, +10 because see invisibility is specially designed to counter invisibility). There is little chance the sneaking wizard will make it past unseen.
Example 2: Suppose the 5th level wizard casts dispel magic, catching that invisible 3rd level wizard in the area of effect. Because dispel magic has rules to deal with spell interactions that includes a roll, the “Spell Effect vs. Spell Effect” rules do not apply.
Example 3: Suppose the sneaking wizard of Example 1 is 14th level instead of 3rd, and casts greater invisibility instead. Like above we would use the “Spell Effect vs. Spell Effect” rule. The greater invisibility‘s check is 1d20+18 (+14 for caster level, +4 for spell level); the see invisibility’s check is still 1d20+17. The higher level wizard, using a higher level spell, has almost even odds of making it by undetected.
Example 4: Now suppose the detecting wizard of the previous example is 14th level and using true seeing instead of see invisibility. The greater invisibility‘s check is still 1d20+18; however, the detecting wizard’s spell true seeing now has 1d20+30 (+14 for the caster level, +6 for the spell’s level, and +10 because true seeing’s description states that it overcomes all types of invisibility ). The sneaking wizard is again in trouble.
Example 5: Continuing the trend, suppose that the sneaking wizard had also cast nondetection. True seeing attempts to interact with the greater invisibility, at which point the nondetection interferes. Because nondetection has rules for how it interacts with divinations2, the “Spell Effect vs. Spell Effect” rule does not come into play unless true seeing triumphs over the nondetection. In this case, see Example 4 above.
Example 6: A 1st level good cleric is facing off against a 17th level evil wizard. The cleric casts protection from evil on himself right before the wizard casts dominate monster upon the cleric. Normally protection from evil would completely negate the effect; because of this, the “Spell Effect vs. Spell Effect” rule comes into play. In the opposed checks the protection from evil has 1d20+12 (+1 for caster level, +1 for spell level, +10 because protection from evil specifically states that it prevents the use of mental control); dominate person, on the other hand, has a whopping 1d20+26 (+17 for caster level, +9 for spell level). Suppose the cleric rolls extremely well, getting a total result of 30, and the wizard rolls very poorly, also getting a total result of 30. It’s a tie! In this case the wizard’s dominate monster wins the tie, because that was the spell that initiated the conflict.
Example 7: What if the 17th level evil wizard from the previous example has cast dominate monster onto the good cleric’s barbarian friend? The cleric casts protection from evil onto the barbarian as he passes. Just as in the last example, protection from evil rolls 1d20+12 versus dominate monster’s 1d20+26; just as in the last example, the cleric rolls extremely well for a result of 30, and the wizard rolls extremely poorly for a result of 30. In this case the tie goes to the cleric. The barbarian gains all of the benefits from the protection from evil.