For reference, some effects that do ability score damage include:
Wounding weapons (1 Con damage per hit)
Crippling strike (2 Str damage per sneak attack)
Green slime (1d6 Con per round, no save)
Monster attacks (Variable , sometimes no save)
Poisons (Various damages, save negates)
Spells e.g.
Poison (1d10 Con, Fort save negates)
Lahm’s finger darts (1d4 Dex per dart, max 5 darts, no save)
Cloudkill (1d4 Con/round, Fortitude half)
Prismatic spray (Death/Fort save partial for 1d6 Con)
Slimewave (1d6 Con per 5 ft of face, Reflex negates)
There are also spells like Feeblemind, Ray of Enfeeblement, Entangle, and Waves of Fatigue that give penalties to ability score, but not actual drain. These generally don’t cause any game imbalance since such penalties don’t stack, so I’ll stick to discussing ability damage only.
Basically, a character tends to be exposed to increasing amounts of damage as they go up level. While their hit points increase constantly, a character’s ability scores do not tend to increase as much- generally a character’s ability scores (other than their primary attribute: Dex for a rogue, Str for a fighter, Int for a wizard, etc.) will be unchanged except for possibly an inherent bonus and an enhancement bonus. Monster characters with much higher attributes do tend to proliferate at higher levels, (but they shouldn’t be used in any game balance example).
Consequently, a high level character automatically have an Achilles heel: it becomes much easier at high level to deal 20 points of Constitution damage than it does to inflict 200+ hit points of regular damage. With the exception of constructs, undead, and deities, very few creatures are immune to ability score damage, and there are no spells that I know of that protect against it in general. Even if there were, it would still not necessarily be damaged.
Many forms of ability score damage allow saves, but not all. Characters should have some vulnerabilities and its fine for ability score damage to be scary, but it should not be as overused at high levels as it currently is (many to most epic monsters inflict ability damage), and the most broken excesses should be curbed.
I’ll now discuss some problem areas individually.
Wounding Weapons
These are particularly nasty since they inflict Constitution damage. Since it’s a weapon, a character is quite likely to be hit by more of them at higher level (since characters gain more attacks a round). In game AC tends to lag behind attack bonus, and characters or monsters can also increase their number of attacks with haste, expert tactician, two-weapon fighting, multiweapon fighting and attacks of opportunity for reach, etc so characters will be hit with more attacks.
Since ability scores don’t scale as well as HP, they will therefore be exposed to more ability damage. There is no intrinsic reason why arrows of wounding could not exist (though they’re technically not legal, currently): a few low-level archers could do an enormous amount of Con damage. Other than their AC, nothing stands between the unfortunate PCs and a horrible and squishy death.
Solution: allow a Fortitude saving throw (DC 20) to avoid Con damage from wounding. This might reduce it to roughly a +1 market price modifier. To prevent rolling huge numbers of saves, roll one save and use for all the wounding damage.
Crippling Strike
The rogue ability Crippling Strike has similar abuses: 2 strength damage per sneak attack can add up quite quickly for low-Strength monsters (such as wizards or sorcerers). Used in combination with Waves of Exhaustion and/or Ray of Enfeeblement, this could be used to drop some creatures quite quickly.
Solution: This is less abuseable since there are various ways of avoiding sneak attack, and since this should be worthwhile (being a high-level class ability), but even so I believe crippling strike should either allow a saving throw, or be useable only once per round. If the save, I’d use a DC of 10 + rogues’ number of sneak attack dice + Dex modifier.
Green Slime
Possibly the worst example. This horrific substance devours flesh, so why it doesn’t simply do HP damage like (say) being immersed in acid, I don’t know. While as a plant on ceilings it’s easy to avoid, imagine a high level fighter (maybe with two-weapon fighting) throwing flasks of it at their enemies. Even worse is the Slimewave spell in Masters of the Wild: this hits a larger creature with multiple patches of slime simultaneously. Since Con doesn’t increase as fast as monster surface area, this spell is a mild inconvenience to Medium creatures but basically devours Colossal monsters whole. Also, since it damages organic substances it should work on zombies etc. I rest my case.
Solution: have green slime do hit point damage (say 3d6 per patch)
Poison
Poison can be unbalanced even with a save allowed, since multiple doses are cumulative. Once when GMing a 9th level party two characters from the same player (one monk/assassin, one minotaur) died from black lotus venom in consecutive game sessions- given a low Fortitude save, multiple applications of a high DC poison are deadly. Which probably accounts for the extreme and massively inflated price that most poisons have.
On the other hand, in high-level games poisons are virtually useless, since the save DCs of poisons do not generally keep up with character’s increasing fortitude saves. Adding in the effects of spells like neutralize poison, delay poison, and periapts to protect against it (since 3.5 amulets grant complete poison immunity, rather than +4 to save) and poisons quickly become worthless.
Solution: Limit a character from taking damage to a particular ability score from poison at most once per round, but increase the save DC for a larger dose, see below.
Variant Rule: Multiple Poison Attacks
*If a character takes several poison attacks simultaneously roll a single save, increasing the save DC of the poison by a +2 circumstance bonus per additional dose.
*If they are poisoned and fail the save, ignore later poisonings that round since a character can only take damage once per round.
*If they are poisoned but save, then are poisoned again later, roll another save to check for damage (with no circumstance bonus for the first poisoning).
A check for secondary damage is always treated as several simultaneous poison attacks.
If you use this variant rule, I suggest you reduce the listed cost of poison by 50% as it makes poison less harmful.
Spells and Special Attacks Dealing Ability Damage
The worst offenders here is Lahm’s finger darts (Book of Vile Darkness: 1d4 Dex damage per finger dart with no save, caster loses 1 str per finger dart, maximum 5 darts). Repeated use of this spell can wipe out a PC of any level. One application of this spell effectively destroys a Great Wyrm Red Dragon (CR 24, sadly only 10 Dex) if you get through SR.
Another is Cloudkill, as its a fortitude partial effect. Forcecage the 30th level dwarf fighter with 550 HP and fort save +36 within a cloudkill spell, and he’ll be dead in under 2 minutes.
Solution: one solution would be to always allow a saving throw for no ability damage from spells and special attacks dealing ability damage. Another solution would be to allow Fortitude saves that critically succeed (10 or 20 over the listed DC to save, as the GM prefers) always ignore partial damage.
Ability Damage and Enhancing Items.
It should also be clarified what happens when a character is ability damaged and has an ability score bonus from an item. For example if my wizard has 8 base Str and a Bull’s Strength cast, then takes 8 points of strength damage from shadows, is he:
A) Dead, because his base strength is 0?
B) alive and cannot be killed, because he can’t lose those last 4 Strength points?
C) alive and has 4 strength left?
Here, I personally believe option C) makes the most sense.
Conclusion
Mostly the ability damage game mechanics are much bettering in 3E than in previous editions, but still need tightening. None of my fix-it ideas here have been playtested, so treat with caution.
Since ability scores don’t scale as well as HP, they will therefore be exposed to more ability damage. There is no intrinsic reason why arrows of wounding could not exist (though they’re technically not legal, currently): a few low-level archers could do an enormous amount of Con damage. Other than their AC, nothing stands between the unfortunate PCs and a horrible and squishy death.
Stirge arrows from AEG are better. 1d4 points of con damage per hit(no save) and they are legal.
Just think of the archer with these. Say he does an average of 2 points of con damage per hit. A balor's con is around 20 IIRC so a regular 16th level archer with these could bring him down in 2 rounds with rapid shot.I do agree that some things need to be changed with this because munchkins just eat this stuff up.
A couple of specific points regarding con loss/drain/damage:
Green Slime: It will devour flesh at the same rate regardless of character level, why should it be more deadly for lower level characters? Higher level characters have had more chances to have encountered Green Slime and, thus, should have more of a chance of realizing how to deal with Green Slime ... or even recognize Green Slime so that they don't foolishly run into it.
Wounding Weapons: At low levels, the PCs are only dealing out an amount of attacks relative to the amount of attacks being directed at them. At higher levels, the PCs are again dealing out an amount of attacks relative to what they receive. At low levels, characters are only likely to see 1 or 2 points of damage from a single wounding weapon and it will take them longer to neutralize the source of that damage. At high levels, while character can take 3 or 4 points of con damage a round form a single wounding weapon, they also have more means to neutralize the source of that damage. By neutralizing, I don't mean protection from the ability damage itself, but rather the ability to stop the wielder of a wounding weapon.
Poison: It is indeed deadly, which I believe the point is. Actually, I think 2e poisons were far more deadly than they are now even. I don't recall seeing any save or die poisons in recent additions of D&D (or at least in the core rules); though I could well be wrong. Still, primary and secondary damage can indeed lead to something similar to a save or die situation for poison. Either the case, this is something like Green Slime, it is just as deadly at low levels as it can be at high levels. Hit Points represent a combination of things which has been argued around the boards, however, hitting vital organs internally, I don't think experience for sustaining damage really helps anyone. Not matter the level, all characters are susceptible to poison, I don't understand why a higher level character should be able to stop poison flowing in their blood stream easier than a low level character.
I didn't address the other abilities because a constitution of 0 is the only ability drain/damage that will kill a character. Any other ability at 0 is really making the character immobile and helpless, not dead.
Ability Damage and Enhancing Items.
It should also be clarified what happens when a character is ability damaged and has an ability score bonus from an item. For example if my wizard has 8 base Str and a Bull’s Strength cast, then takes 8 points of strength damage from shadows, is he:
A) Dead, because his base strength is 0?
B) alive and cannot be killed, because he can’t lose those last 4 Strength points?
C) alive and has 4 strength left?
Here, I personally believe option C) makes the most sense.
Strength damage doesn't kill. ONLY Con damage kills, Dex or Str 0 means you are immobile, Int, Wis, or Cha 0 means you have are brain dead, catatonic, etc.
Brknlf: I'd say that higher level characters are not always going to be able to stop someone with a wounding weapon. At higher level opponents should still be relatively challenging, so that a high level character can't necessarily just get away. It’s possible to surprise and Manyshot someone with so much black lotus venom that they die before anyone even realizes the arrows are poisoned: one unlucky first save and the next 3 saves become quite hard.
If paralyzing a great wyrm straight off with Lahm's finger darts (a 2nd level spell) doesn't convince you, and a balor dropping in a round from stirge arrows doesn't offend you, I really don't know how to convince you ability damage doesn't work.