What happens to your spells if the ability score that governs them is somehow reduced? Do you lose any spells? If your spellcasting ability is reduced to 0 or less do you lose all your spellcasting ability?
Some of these questions are easy to answer, and some are trickier. Let’s start with the easy answers:
No ability score can fall below 0, and there’s no such thing as a negative ability score. If ability damage or other reduction would reduce an ability score to less than 0 ignore the excess loss or reduction, it’s irrelevant to the game.
All abilities that govern spellcasting (to date at least) are mental: Intelligence, Wisdom, or Charisma. When any of these three abilities fall to 0, you’re unconscious and you can’t do a thing, least of all cast a spell.
The rules don’t have much to say about the remaining questions here, but one can make some educated guesses.
You must use your reduced ability score to determine the highest level spell you can cast. Any prepared spell, or spontaneous spell slot, that’s currently too high level for you to cast isn’t available to you until your spellcasting ability score improves. A prepared spell is not lost and a slot for spontaneous casting is not used up (but heed the following about bonus spells and spell slots).
Use your current (reduced) ability score to determine your bonus spells. Count your spells after you do this. If your current set of available spells at any given spell level has more spells (or slots) than you can carry with the bonus spell reduction you lose the excess spells. I recommend allowing the afflicted spellcaster to choose which spells become lost (think of it as a mental struggle to hold onto the spells as they waver and slip away).
Once you lose a bonus spell or spell slot, it’s just as though you had cast the spell. The slot is empty for the day. You can’t put another spell into it—even if your spellcasting ability later improves—until you rest and renew your daily spells again.
While your spellcasting ability score is reduced, you must use your reduced ability modifier to determine saving throw DCs for the spells you cast, and for any other variable that depends on that ability score.
Ability score losses can be a royal pain for players and DMs alike. Players have to fiddle with the numbers on their character sheets and DMs have to make sure everything gets handled correctly. After you handle the paperwork, everyone still must contend with the effects from the loss.
Nevertheless, ability loss is an excellent tool. A minor ability loss usually hampers a character without being deadly—the character feels the effects, but isn’t in any real danger. Things merely become a little harder to do, and that can teach even the most dominant character a lesson in humility. It also can force players to seek new solutions to problems that have become routine for them.
One great thing about ability loss is that it affects characters of any level more or less equally. High-level characters tend to have slightly higher ability scores than lower-level characters, but not so much higher that they can ignore losing ability points a d6 at a time. Also, even a low-level character can take an ability hit and keep functioning, even if the character feels a bit wobbly at the knees.
This is one reason why effects such as poison and disease deal ability damage—they hit everyone more or less equally that way.
Got a question for the kobold? Give us the stumpers in comments!