Quote Originally Posted by St Fan View Post
Q 92: If this spell is timed to strike a spellcaster while trying to cast a spell (with a ready action), must the target make two separate Concentration checks with different DC (one to avoid attack of opportunities, one to not flub the spell)? Or would the second check (to not flub the spell) only be required if the first is failed?
A 92

The targeted spellcaster must always make at least two Concentration checks, but the first Concentration check determines how hard the second Concentration check (or checks) will be.

When you ready the act of casting the Slapping Hand spell to interrupt an enemy's spellcasting action, we must (following the rules of readying) first resolve the interrupting action (the Slapping Hand spell) and then resolve the interrupted action (the enemy's spellcasting action).

Thus, the enemy spellcaster must first make a Concentration check at DC 20 to shake off the effect of the Slapping Hand spell. If this Concentration check fails, the spellcaster unavoidably provokes attacks of opportunity, because this is what the Slapping Hand spell causes to happen. Despite being attacked, the spellcaster can still try to cast their spell, but they can succeed only if (1) all attacks of opportunity miss or (2) the spellcaster, after each successful attack of opportunity, both survives the attack and makes a Concentration check whose DC is 10 plus the spell's level plus every Hit Point of damage taken.

If the enemy spellcaster's Concentration check at DC 20 succeeds, they shake off the effect of the Slapping Hand spell. Of course, casting a spell still provokes attacks of opportunity by default. However, the enemy spellcaster, having shaken off the Slapping Hand spell, can avoid attacks of opportunity in the usual way, namely by casting defensively (that is, making a Concentration check at DC 10 plus the spell's level). This is a much easier Concentration check to make, especially if the enemy spellcaster has the Combat Casting feat.*

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*As I understand the rules, the Combat Casting feat adds a +4 bonus to only one of these Concentration checks: the one that the enemy spellcaster makes to cast their spell defensively. Neither the Concentration check to shake off the effect of the Slapping Hand spell nor the Concentration check to cast a spell despite being injured can add this +4 bonus. Thus, readying the Slapping Hand spell against an enemy spellcaster, if it succeeds, prevents the enemy spellcaster from using the Combat Casting feat.