My "Shackled City" campaign will be wrapping up in the next two months, just in time to start an "Age of Worms" campaign. I'm in the process of trying to decide on house rules for the upcoming campaign.
One of the two things I'm most dissatisfied with in 3.5 D&D is the rule set for concentration while spellcasting. (The other is turning undead.)
Playing by the standard rules, it becomes way too easy at higher levels to disrupt a spellcaster if you ready an action and attack while the spellcaster is casting.
For example, suppose you have a 17th level caster, who has maxed out his ranks in concentration and has a +2 constitution bonus. This gives him a concentration check of +22. If the caster is struck in combat while casting a spell, the DC for the concentration check is 10 + damage dealt + spell level.
If he is struck for 32 - spell level points of damage, which is not uncommon at high levels, there's no way he can succeed on the concentration check.
The problem is that a concentration check based on damage dealt doesn't scale properly. The skill check goes up by one point per level, but the typical damage dealt goes up by more than one point per level. (Esp. with spellcasters, where typical damage dealt goes up 1d6 or 1d8 points/level for wizards and sorcerers, or 1d6 points/2 levels for clerics and druids.
I see two ways of remedying this problem. One is to make the DC for the concentration check dependent on something other than damage, something that advances about 1 point/level, or to drop the concentration check mechanic altogether, in favor of something that does scale with damage dealt.
Option 1:
The DC for a concentration check when a character is hit by damage is determined not by damage dealt, but by the BAB of the attacker, plus the attackers combat ability modifier (strength or dex, or sometimes some other ability, e.g. if the attacker has Zen Archery). If the caster is damaged by a magical attack, the DC is based on the caster level of the attacker (or the magic item), modified by the attackers (or item's) relevant ability bonus. The DC for a concentration check is 10 + attacker's BAB or caster level + attacker's relevant ability score + spell level.
Using this rule, a fighter of level L with str of N would have a 50% chance of disrupting a 1st level spell cast by a caster of level L with a con of N. Seems about right to me.
Advantages: maintains the d20 mechanic.
Disadvantages: it seems like the chance to disrupt should be based on damage dealt; what to do about damage that is not from an attacker, e.g. falling rocks.
Option 2:
Ditch the d20 mechanic altogether.
If a caster with M hit points is struck in combat for N points of damage, there is a N/M chance of disrupting the spell. The casters hit points are his total hit points, not counting any damage or temporary hit points, but counting any adjustments due to constitution buffs or damage.
So if a caster with 40 hit points (not counting any damage or temporary hit points) is hit for 20 points of damage, there is a 50% chance of disrupting the spell he is casting.
Since a caster's hit points (1d4 or 1d6 + con per level) will scale at about the same rate as damage dealt by attackers, this works out.
What to do about casting on the defensive? Make it a straight caster level check DC 10 + 2 time spell level. So at whatever character level a wizard gets a new spell level, he has a 50% chance (a roll of 11 or better) of successfully casting spells of that level on the defensive. (Slightly less for sorcerors.) Combat Casting feat gives you a +4 on this check. Something similar for casting in storms and such.
Advantages: chance of disruption still tied to damage dealt; frees up one skill point/level for spellcasters, allowing them to become more diversified.
Disadvantages: not a d20 mechanic; requires getting out a calculator; chance of having spell disrupted not dependent on spell level (that could be factored in, but it's not obvious how to do that, and the mechanic is complex enough already).
Comments?
Here's another option.
Change the DC from:
10 + damage dealt + spell level
To:
10 + damage dealt
This would ameliorate the scaling problem to some degree, but not completely.
ok, first if a caster of 17th level gets engaged in combat then the player has messed up, but think of it this way, it is supposed to be easy to disrupt a spell by hitting the caster thats the point its all the fighter has, taking that away is a big unballance toward casters. A first level caste maxed out in concentration with say a +2 mod has a 5 total to his check, while a 1st level fighter say with a long sword hits for an average of 4 with say a +3 strength, so 7 which means tha DC at first level on average is 10+7+1 an 18, so the caster needs a 12 or higher a 45% chance, it stands to reason that as both progress that a higher skilled fighter would have an easier time disrupting a caster doing a more difficult spell, I think you should reconsider changing this, but its your game and if the players are ok with it then go for it
The debate about spellcasters and their Concentration checks has been going with my group since 3.0 started. Some of my players suggest that any spellcaster damaged during the round he casts a spell should make a Concentration check. Of course, these guys never play spellcasters themselves and get annoyed when NPC spepllcasters cast spells on them after they have been hit for damage. My other players (who play spellcasters on a semi-regular basis) have no problems with the system as it is, but get equally annoyed when my NPCs take readied actions to try and disrupt their spells. You just can't win.
Readying an action to disrupt a spellcaster is not always the easiest thing to accomplish, especially when spellcasters are savvy to the concept, and they can often position themselves in such a way as to avoid attacks. Also, at high levels readying an attack against a spellcaster means that characters are foregoing the ability to make multiple attacks (since you can't ready a full-round action). You should remember these points before changing the rules.
if you are going to alter the rules, try to choose something that uses the D20 rules system and doesn't make the Concentration skill obselete. Perhaps you could create a feat that would allow a spellcaster to use his Intelligence, Wisdom, or Charisma when determining the bonus to his Concentration checks. The spellcaster would be more likely to succeed at higher levels (as his stats increased) and you would not have to alter the current system.
In the end make sure that your players accept the rules and understand the reason behind the change. If your players hate the fact that the rules have been changed then you probably shouldn't do it, but if they agree then go right ahead!
In all honesty, have you ever tried completing a math puzzle after just banging your head? When some is wounded in combat, it surprises me that they are able to keep on their feet, let alone continue the complex evocation of a spell. Still, to keep the game balanced and allow spell-casters to have a chance, the concentration check exists. I would see little point in dropping or changing it.
Ever been in martial arts? :-) In the frenzy of combat, you'd be amazed what you can do after getting hit, and hit hard. Honestly, I have no problems believing in the idea of a spellcaster being able to maintain their concentration despite smacked around a little. :-)
As for scaling the DC by damage... It makes no sense. The whole idea of HP is that you're not really getting tougher with higher levels, you're just getting better at taking the damage in less-critical ways. A 1st-level fighter taking 14 HP damage probably has a sword through the belly. A 10th-level fighter with a sword through the belly is just as badly injured, but the difference is that a 14HP damage attack doesn't skewer him, because he's got the experience to twist a little and turn the skewer into a nasty gash along his abdomen. (Falling is one of those areas where this falls apart, but that's OK. ;-) )
What might work better would be to make the damage modifier dependent on the percentage of the spellcaster's HP. This involves a lot more math... But what I'd probably do is N/M * 20, where N is the damage taken in the attack and M is the caster's total hit points. So if a caster with 40HP took 20HP damage, the modifier is 20/40*20 = 10. A caster with 4HP (first-level sorcer with no CON bonus) who took 3 HP damage would have the concentration check modified by 15, for a total of 26 -- which is nearly impossible for a first-level caster to beat. This makes concentration a lot less valuable at low levels, but it makes it very valuable at high levels. It also gives the players a lot more incentive to cast defensively, and gives the party a lot more reason to protect their spellcasters. It does involve a lot more math... For example, what's the DC modifier for a 17HP wizard taking 5HP damage? 5.9, rounded up to 6. But try doing that in your head in the middle of combat without slowing down the game... :-)
In all honesty, have you ever tried completing a math puzzle after just banging your head? When some is wounded in combat, it surprises me that they are able to keep on their feet, let alone continue the complex evocation of a spell. Still, to keep the game balanced and allow spell-casters to have a chance, the concentration check exists. I would see little point in dropping or changing it.
There is actually very little to remember when casting a spell. In the case of clerics, druids, and wizards (Im not sure about bards and sorcerers), most of the spell is prepared beforehand and when the spellcaster releases the energy for the spell he or she does so with a simple gesture, a couple of spoken words, etc. That is why spells are so quick to cast and spellcasters don't have to remember the equivalent of complicated mathematical formulas, etc. At least that's how designers like Skip Williams and David Noonan have put it in the past, and they helped design the 3.5 rules.