Mage Slayer revised.

[Joker]

Mage Slayer

Your training has given you a strong will and the skill to harass spellcasters.

Prerequisites: Bab +1, Spellcraft 3 ranks.

Benefit: You gain a +1 on all Will saves. Additionally, if a spellcaster tries to cast a spell defensively in your threatened area you add your Base Attack Bonus to the Concentration check DC. Also, if you have identified the spell being cast, the DC increases by another 2.

Special: Note that unlike the original Mage Slayer feat (CA), a spellcaster doesn't mystically know you have this skill. He may learn in time but that is up to the DM. Though, I suppose, if you get drunk alot and tell everyone you're the one and only 1337 mage-whacker, your future opponents might use different tactics. DM's discretion.

 

[the Jester]

This is much better than the original, 'better than an epic feat' Mage Slayer imho.

Good work.

 

[Brain]

This is much better than the original, 'better than an epic feat' Mage Slayer imho.

Good work.

Although this version is still better than the epic feat, I agree that this is a step in the right direction.

 

[Cheiromancer]

Mage Slayer

Your training has given you a strong will and the skill to harass spellcasters.

Prerequisites: Bab +1, Spellcraft 3 ranks.

Benefit: You gain a +1 on all Will saves. Additionally, if a spellcaster tries to cast a spell defensively in your threatened area you add your Base Attack Bonus to the Concentration check DC. Also, if you have identified the spell being cast, the DC increases by another 2.

Special: Note that unlike the original Mage Slayer feat (CA), a spellcaster doesn't mystically know you have this skill. He may learn in time but that is up to the DM. Though, I suppose, if you get drunk alot and tell everyone you're the one and only 1337 mage-whacker, your future opponents might use different tactics. DM's discretion.

I think it might scale a bit too fast. A 3rd level fighter with this feat squares off with a 3rd level wizard. The wizard casts a 2nd level spell defensively, but the fighter doesn't identify it. The concentration DC is 15 + 2 (spell level) + 3 (BAB) = 20. The wizard has a 14 Constitution and max ranks in Concentration (6), so has to roll a 12 or better to make the check.

A 20th level fighter squares off with a 20th level wizard. The wizard casts a 9th level spell defensively, and again the fighter doesn't identify it. The concentration DC is 15 + 9 (spell level) + 20 (BAB) = 44. The wizard has bear's endurance and so has an 18 Constitution, and has kept his Concentration score maxed out (23 ranks). He needs a 17 to make the check.

See the problem? The wizard has gone from having a 55% chance of getting the spell off to having a 20% chance. I suppose he could have taken Combat Casting and Skill Focus (Concentration), but that's hardly typical; wizards have better things to spend their feats on. He might not even have kept his concentration score maxed out. 20 ranks would have been enough to ensure defensive casting in normal circumstances. And the fighter might have made the spellcraft check to identify the spell.

The original version gave the wizard warning not to try to cast defensively, but this one doesn't. The original version also penalized the magical ability of characters with this feat (-4 to the caster level of spells and SLAs) but this one doesn't. If the only other difference between this feat and the CA one is that this feat only reduces the chance to cast a spell by 80% instead of 100%, then this feat seems a bit too good.

Perhaps you could add in a DC 15 spellcraft check for the wizard to notice that the fighter's stance is perfect for disrupting spells cast nearby. If the wizard has observed the fighter in combat, the fighter's BAB is added as a bonus to the spellcraft roll. This gives an in-game explanation for the wizard's "mystical insight."

I don't know what to do about the caster level penalty. I don't know what the reasoning is for it, but it certainly makes the CA version less attractive; it serves as a warning that the benefit has to be counterbalanced by some pretty hefty penalties.

 

[Joker]

I wanted to create the option for PC's and NPC's alike to at least have a chance to disrupt a spellcasters spell. Without the feat the concentration check to Cast D. is ridiculously simple and at some point isn't even nescessary.

It is true that this feat makes casting defensively very hard but instead of giving the defender a check to beat I would prefer to leave it to DM's discretion because he knows how observant the defender is.

 

[the Jester]

One fairly elegant solution is to just use 10 + BAB + spell level for the DC. Even then, at higher levels it becomes more difficult to cast on the defensive, but at higher levels most casters aren't directly threatened as much (they tend to have much better mobility, summoned bodyguards, etc).

 

[IndyPendant]

Well, one thing I'm curious about: why is it so bad that a caster not be able to cast defensively? So long as he knows about it in advance (which is, I assume, why that flavouring was added about the caster knowing about it), he's not going to waste an action. All he's going to do is risk a single AoO--or (in nearly every case) simply 5' step out of the threatened area and cast.

There are like 92387421984 ways for a caster to easily cast a spell while threatened. So much so, that it's become a caster's "right" in people's minds, rather than a risky maneuver. I'm not sure I agree with that.

However, I do like the mechanic of identifying the spell. I think I may try to add that in to my own version of the feat--which is considerably less harsh for the person taking it than in CA. However, Spellcraft is a cross-class skill for the ones that would be taking this ability most often (tanks and monks), so they would never be able to succeed even half the time. I'd have to make a few changes before introducing that into the feat.

Anyways, here's the version I've made available to my players. No one's taken it yet.

Mage Slayer [General]
Source: Adapted from Complete Arcane.
Prerequisites: Spellcraft 2 ranks.
Benefit: Spellcasters you threaten may not cast defensively (they automatically fail their Concentration checks to do so.) Spellcasters are aware of this limitation when being threatened by a character with this feat.
Special: Unlike in the Complete Arcane, this feat does not require BAB +3, you don’t gain a +1 Will save bonus, and does not reduce your caster level by 4. Those modifiers ruin the feat, and have been eliminated.

 

[Greyskull]

Another option is to simply change the casting defensively rule to the one that appears in Monte Cook's Arcana Unearthed/Evolved.

Casting Defensively
If the character wants to cast a spell without provoking any attacks
of opportunity, she needs to dodge and weave. To cast defensively,
she must make a Concentration check opposed by the foe’s attack
bonus (or 10, whichever is higher) + the level of the spell she is
casting. The character loses the spell if she fails this check.

 

[Cheiromancer]

Another option is to simply change the casting defensively rule to the one that appears in Monte Cook's Arcana Unearthed/Evolved.

Casting Defensively
If the character wants to cast a spell without provoking any attacks
of opportunity, she needs to dodge and weave. To cast defensively,
she must make a Concentration check opposed by the foe’s attack
bonus (or 10, whichever is higher) + the level of the spell she is
casting. The character loses the spell if she fails this check.

I might have gone BAB instead of attack bonus, but I guess this is better, especially at medium levels, when weapon enhancement, weapon focus, enhanced strength and so on really start to push the attack bonus up. If you went the AE/E route, what benefit would a feat add? +4 to the DC of the caster level check?

 

[Joker]

Another option is to simply change the casting defensively rule to the one that appears in Monte Cook's Arcana Unearthed/Evolved.

Casting Defensively
If the character wants to cast a spell without provoking any attacks
of opportunity, she needs to dodge and weave. To cast defensively,
she must make a Concentration check opposed by the foe’s attack
bonus (or 10, whichever is higher) + the level of the spell she is
casting. The character loses the spell if she fails this check.

This is our group's current house rule on this matter (minus the addition of the spell level to the DC). And in my experience the attack bonus are much greater than the skill modifiers.
One time when fighting a dragon of our CR it was so that we could only cast defensively if he rolled a one (counts as -10 in our campaign) and we rolled 15 or higher.