Introduce CoC d20's spell costs into DnD3.x

Question

Call of Cthulhu d20 has a Sanity point loss and/or (temp.) ability drain cost for spellcasting. How would it be best to introduce this system into DnD3.x?

Are there (semi-)official guidelines for this? If not, how would you do it? Would you introduce Sanity as well, or replace it with something else (hit points, for example)? Would you set up a general, level based cost for spells, modified by school and/or the DM's whim (...I mean, based on actual circumstances, spell effect, spell target etc)? How would you rebalance the spellcasters' power level compared to the other classes? Would the cost system apply to divine spells as well?

Seems like there are quite a number of sub-questions - but they're just that, sub-questions (to be carefully considered) of the main one, which is, once again:

How would you introduce the spell costs of Call of Cthulhu d20 into DnD3.x?

Bonus points for considering the adaptation to Ravenloft 3.5's rules as well (as an addendum to the generic DnD3.x ones.) ;)

Important edit: Please, do not recommend using a different system (even if it has similar effects) only. Feel free to mention such, of course, if you think it's better than what I'm considering, but do focus on and include your recommendation for the CoC d20 + DnD3.x rules fusion. Thank you. :)

Answer 1

Heroes of Horror has Taint rules. They’re not automatically associated with spellcasting (outside of specific spells added by Heroes of Horror), but they could be.

I don’t recommend that you do, however. The Taint rules are pretty bad. They’re complicated, and utterly debilitating in even small amounts, unless you use a couple of tricks (namely, be Undead and take the Tainted Scholar prestige class) that turn them into one of the most overpowered resources in the game.

Unearthed Arcana has a simpler sanity system. I don’t know a lot about it, but in reading it, it feels really tacked on. Can’t really recommend them, though they look better than the Taint rules. Again, they do not, by default, apply to just normal spellcasting.

I’d be dubious about a straight port from Call of Cthulu though. Dungeons & Dragons 3.5 is one of the highest-magic systems I’ve seen, and the system by and large assumes that magic is readily available for handling many encounters, especially once you leave low levels. While magic is massively overpowered, and could definitely use a nerf, you need to be very careful about it.

As a result, I don’t think any simple, tacked on system is likely to succeed very well, whether it be from Call of Cthulu, Heroes of Horror, or Unearthed Arcana. Unfortuantely, due to the sheer number of spells available, going through each one and figuring out its intrinsic risk or cost is not feasible.

So what I recommend is that you try one of these systems, or some system inspired one or all of them, but then you test it a fair bit before you attempt to use it in a campaign. Try to find someone who knows how to play Wizards well, and run a Wizard 1, Wizard 5, Wizard 10, Wizard 15, and Wizard 20 through various challenges. Probably best to give him a few beatstick allies to allow him to take advantage of his buffing spells.

This will probably take a while. You need a variety of different types of challenges, things that require spells, and you need to test a fair few levels. But I think it is the only way you can develop a system that is actually fair.

Answer 2

I don't know CoC d20, but I do know the BRP version. In it, most of the spells are not useful in combat; they are used outside of combat to summon monsters, banish things and so on.

Therefore, you could leave 3.5s magic system alone, and have a separate 'ritual' system using the CoC spells. 4e rituals could be a guide; anyone can use them, but they have costs (Money, sanity, taint, whatever) and need skill checks.

Sanity rules have been ported to D&D in a variety of forms, officially as KRyan talked about, or in a simpler version by Kobold Quarterly. I've found one article on the blog about it, but likely you'll have to seek out the issue (#11) of the magazine for the full thing. These are pretty much directly copied from the BRP rules, so they should be pretty compatible with CoC magic.

There is also a previous question on importing sanity rules to d20 that you might find helpful.

Answer 3

If you don't mind the bookkeeping, get an idea of the cost per level of the CoC spells by looking at the conversions at the back of the book, and choose the ability to drain in D&D that most closely interacts with the spell in question. Or just roll a d6 and drain a random ability.

Answer 4

Unearthed Arcana (3.5) has a cost-based spell system as an alternative, optional rule. I believe there is also a Stamina system. Also, d20 Modern campaign setting has a low magic setting, which may be more appropriate to what you are looking for as far as balance goes.

Review those rules, and apply the ones that most closely match your perception of the CoC ruleset. This would allow you to implement the CoC system using pre-existing 3.x rules.