Counterspells of Opportunity

The rules, as they currently stand, make counterspelling a pretty worthless option. You get to hold your action, when you could be casting a potentially devastating spell, in return for the chance to cancel an enemy spellcaster's spell, provided you can pass a Spellcraft check and have the same spell memorised? Better simply to unleash that fireball, and take the reponse like a man.

I would prefer to see counterspelling using much the same rules as attacks of opportunity, as follows:

Counterspell

Whenever an enemy spellcaster casts a spell, you may make a Spellcraft check (DC 15 + spell level). Success indicates that you correctly identify the spell. You then have the option to counterspell. Counterspelling requires that you cast an abridged version of the same (or countering; see below) spell to negate the spell being cast.

You may counterspell any spell with the same spell. You may counter any spell with dispel magic or greater dispel magic. If you have the Improved Counterspell feat, you may counter any spell with a higher level spell of the same school. Finally, some spells specifically state that they counter certain other spells, as noted in their description. Metamagic feats have no impact on whether a given spell will counter another spell - a heightened spell will counter a normal version of the same spell, and will be countered by a normal version of the spell.

To counterspell, you must be able to cast the requisite spell. For instance, a wizard who wishes to counter a fireball must have fireball memorised. A sorcerer must know fireball, and have a spell slot of 3rd level or higher free to cast it. Additionally, you must fulfil all the component requirements for the spell - you must be able to speak to counter a spell with a verbal component, must have at least one hand free for spells with a somatic component, must have any material or focus components already in hand, and must spend any required experience points. However, the casting time for a counterspell is free; that is, you can instantly counterspell a spell with a casting time of ten rounds, for example.

Finally, counterspelling takes the place of making an attack of opportunity. That is, you cannot make an attack of opportunity in the same round as using a counterspell. A caster with the Combat Reflexes feat cannot make any attacks of opportunity in the same round as using a counterspell. You can counterspell only once in the round (but see below). A character cannot counterspell while flat-footed, or otherwise unable to make an attack of opportunity.

Once the character has fulfilled all the requirements for a counterspell, he must make a counterspell check to successfully negate an opposing spell. This check is a d20 roll, to which he adds his caster level (there is a maximum bonus of +10 or +20 when counterspelling with dispel magic or greater dispel magic, respectively. This does not apply to other spells). The DC for the check is 11 + the caster level of the opposing spell.

For instance, if Malgo, a 6th level Sorcerer, is casting fireball, and Ralf, a 9th level Wizard wishes to counter, Ralf must make a counterspell check (d20 + 9), against a DC of 17 (11 + 6).

It is possible to counterspell spell-like abilities, to counterspell using spell-like abilities, to counterspell spells cast from items, and to counterspell using spells cast from items. Spell-like abilities are countered just as would be a spell of the same name. Likewise, they counter spells of the same name. Items are countered by spells of the same name, as normal. They counter as spells of the same name, but can only be used in this manner if the item is held in the user's hand prior to counterspelling - there is no time to ready the action before use (in the same way you can't ready a material component before counterspelling - you're either ready or you can't counterspell). Either way, the caster level for spells cast from an item are determined by the item, not by the character using the item. Counterspelling with an item uses a charge as though using the item normally. Likewise, counterspelling with a spell-like ability uses one of the character's uses per day.

Edit: Oops, it turns out that spell-like abilities can't be used to counterspell, and aren't themselves subject to counterspelling. I also suspsect (although can't find the ruling anywhere), that you can't counterspell with a magic item (although you probably can counterspell a spell cast from a magic item).

So, if Malgo is casting fireball, and Ralf wishes to counter with a scroll of dispel magic (caster level 5th), Ralf must make a counterspell check (d20 + 5) against a DC of 17.

Feat: Reflexive Counterspell

You can respond quickly and repeatedly to opponents who cast spells in combat.

Benefit: When foes cast spells in combat, you may make an additional number of counterspell attempts equal to your Intelligence bonus. For example, a wizard with an Intelligence of 15 can make a total of three counterspell attempts in 1 round - the one any character is entitled two, plus two more because of his Intelligence bonus. You can still make only one counterspell attempt per spell cast by the enemy. With this feat, you can also make counterspell attempts while flat-footed.

Normal: A character without this feat can make only one counterspell attempt per round, and can't make counterspell attempts while flat-footed.

Counterspells and Attacks of Opportunity

In the same way that making an attack of opportunity can provoke an attack of opportunity itself (if, for instance, you choose to disarm), so too does counterspelling. Basically, if casting the spell itself under those conditions provokes an attack of opportunity then using that spell as a counterspell does to (for instance, if you counterspell by casting a fireball, that provokes an attack of opportunity. If you counterspell by casting fireball from a wand, however, that does not).

What this does is adds another level of tactical thinking to combat - the wizard will want to guide his allies into position to attack the opposing spellcaster, so that said spellcaster can't shut him down without suffering attacks of opportunity. I don't consider this to be a bad thing, although it drops the party wizard in the shit if the enemy spellcaster gets there first.

Of course, it would be possible to counterspell defensively, requiring a Concentration check as normal.

Consequences of this Change

Adding these rules has a fairly significant effect on play. Firstly, it makes counterspelling a much more attractive option, since doing so is less of a gamble, and much less costly. Secondly, it can lead to the party spellcasters running out of spells much more quickly, depending on the prevalence of spellcasting foes.

A more subtle effect is that two wizards of the same level are likely to take each other out of the game during combat, as each counters around 50% of the other's spells (actually, rather less unless they've memorised the same selection). This can be a good thing, as it forces each to try to outwit the other, and to use less common spells. It also increases the utility of non-standard, rare and custom spells. If you're the only caster in the world to know Ralf's baleful anger, you cannot be counterspelled easily, whereas everyone knows how to counter fireball. This, then, would encourage characters to seek out lost and unusual spells, which is a good thing.

However, a spellcaster of the same level as the party will not be able to provide a meaningful challenge to the party under this mechanism. The party wizard will effectively shut down the enemy, allowing the party warriors to simply walk over and exterminate him. I'm not sure how different this is from the current state of affairs - lone spellcasters don't do well against full parties.

Finally, this option changes the balance between the Sorcerer and the Wizard somewhat, as the former will have a more potent counterspelling weapon than the latter. This is part of the reason why the Reflexive Counterspell feat relies on Intelligence, to restore parity between the classes.

I would doubt very much that this option would seriously damage any games in which it was used. Counterspelling would certainly become more common, but is unlikely to dominate the game.