The spellcasting system is great in play. Everyone who has played a wizard or cleric has loved it. But it's not easily conveyed in writing, short of copying big chunks of the rules, because (a) there are several parts that interact and (b) the randomization has to be taken in the context of the other random elements of game play. So let me lead into this from a side street...
First, know that the DCC RPG is impossible to power-game, for various reasons, most of them related to the heavy influence of randomization...on everything. I won't say much more than that, but there's no way to min/max this system. What makes it still fun is that the randomization is "bracketed" within certain limits of effect.
For example: there is a critical hit system, which Harley and I have been spending a lot of time working on, which is a ton of fun. Every class has a crit die and crit table for each level. For example, a level 1 warrior has a 1d12 crit die on crit table III, while a level 1 wizard has a 1d6 crit die on level I. As someone advances in level, their crit die gets higher and they may jump to different crit tables. (One crit table, used by thieves and elves, represents finesse. Another, used by 0-level characters, wizards, and halflings, represents lucky but not necessarily lethal blows. Another set, used by warriors in a steadily advancing progression, represents power and strength. And so on.) The combination of crit die and crit tables allows for lots of exciting random results in combat...and emulates pretty well the kind of combat you read about with John Carter and Conan and Elric...but the randomization runs in brackets. Level 4 randomization typically results in "better randomness" than level 1 randomization.
We're still playing a lot with the crit rules, so no more on them now. But that's a roundabout way of leading up to spells, which work in a similar manner. Imagine
magic missile cast by a novice, and then the same spell cast by the world's most powerful magician. Clearly the latter caster should achieve a more powerful result. That's the role of the spell table. Either way you get a
magic missile...but the apprentice with a +1 spell check rolls 1d20+1, and the arch-mage rolls 1d20+12. They roll on the same table, but the apprentice can't get higher than a 21 while the archmage might be rolling a 32. Specific to
magic missile, the lowest result on the table is this:
You can throw 1 missile that does 1 point of damage. You must have line of sight to the target. The missile flows unerringly and never misses, though it may be blocked by certain magic (e.g., magic shield).
...while the highest result is this...
You throw 1d10 missiles that each do damage 1d10 + caster level. The caster may direct these missiles individually as a single action, or may direct them all at a single target who is not present or visible, provided he has specific knowledge of that target. In this case, the caster must have a physical memento of the target (hair, fingernail, vial of blood, etc.) and spend 1 turn concentrating to cast the spell, then continue concentrating as the missiles seek their target. The missiles will aim for this target even if it is concealed or invisible, though they have a maximum range of 100 miles. The missiles will turn, curve, re-trace their route, and make every effort to reach the target, although they cannot cross planes. The missiles can travel up to 10 miles per second provided no obstacles are present, but speed is much lower if, for example, they must navigate underground caverns. Provided a direct route exists, the missiles will strike the target unerringly.
And of course there are many other results in between those two. Magic scales with the caster even on the same spell -- a level 1 spell steadily becomes better and better as the caster advances in level, and a level 1 spell cast by a level 10 wizard can do more damage than a level 4 spell cast by a lower-level wizard. The tables go quite high such that a more powerful caster can achieve more powerful results -- for
magic missile, the highest result noted above requires a spell check of at least +12, which, in general, would require a wizard of level 9 or higher. In other words, there's randomization, but it's "bracketed" -- you always get a
magic missile, but maybe it's better today than the one you cast yesterday, or yours is more powerful than mine...
Beyond that, there are several subsystems. D&D magic is pretty straightforward: you memorize a spell, and lose it when you cast it. This is Vance at his most literal interpretation; his wizards can memorize two, three, maybe four spells, fewer if they're more complex. Gygax took this straight from the source. DCC RPG adds randomization and amplification of results by casters of different levels, both of which are also present in Vance (and other Appendix N authors). DCC RPG also adds elements to capture some of Lovecraft and Robert E. Howard, which were very strong influences on D&D but whose magic systems aren't reflected, as well as Moorcock to a lesser extent. Almost all the magic in Robert E. Howard's Conan series revolves around demonic aid -- there's always a demonic power in the background, aiding the sorcerer, and the sorcerer (always a nemesis of Conan) typically loses his power when that demon's aid is no longer present. This is also evident in Moorcock ("Aid me Arioch! Blood for Arioch!") and of course it's a key theme of Lovecraft. How to reflect that? Well, it's still magic, but with a source, and a source that usually wants something in return. Thus is born the spellburn system, which allows a spellcaster to seek aid from an otherworldly patron -- at a price. A spellcaster can sacrifice physical ability scores (Strength, Agility, or Stamina) in a 1:1 ratio to enhance his spell check. Sacrifice 4 points of strength and add +4 to your next spell check. The ability score loss heals -- slowly -- and there is a table to provide inspiration of exactly what act is committed to cause the loss of that Strength. So your low-level wizard...or, let's say, elf...who appeals to the aid of an otherworldly power...say, Arioch...may get that aid -- but only if he drains his own physical stamina and contributes three souls in the next hour. And of course, at the DM's discretion, that otherworldly power should be role-played, and may come back at a later date asking for something in return! Add it all together, and your low-level wizard may be able to achieve a very powerful
magic missile to save the entire party from certain death -- then he collapses to the ground, exhausted and drained of strength, and has to pay off a favor to a demon someday in the future.
Now mix into all of this what I call "mercurial magic." To quote the manuscript:
The firstborn son of a witch hanged at trial wields black magic adroitly. An orphan raised by satyrs is a precocious student of druidry. Cosmic caprice determines skill in magic: birth order, family lineage, horoscope, and matters even more abstruse have as much influence on a wizard’s spellcasting as his hard work and native intelligence.
And this is where it gets even more fun. Every wizard, for every spell, has a randomly determined "side effect," so to speak. Luck scores come into play here...a wizard with a higher Luck typically has more beneficial side effects. Side effects can vary from random toadstools sprouting nearby whenever the spell is cast, to loud noises or energy bursts. So that same
magic missile in the hands of two different wizards will not only vary based on their casting power, and not only vary based on whether one of them calls to a greater power for aid, and not only vary based on the inherent variability in the table...but will also vary because they each get a slightly different effect when casting it.
I actually had a really fun playtest at one con where a player had a spell that was useless in combat (I forget the exact spell, but it was something like
comprehend languages) but it had the mercurial side effect of causing freezing, damaging cold whenever he cast it. So he would up casting that spell frequently, solely for the side effect, while in combat. It was fun. That is a little more extreme than I originally intended the results to be, but you get the idea...spellcasting varies from caster to caster.
And, finally, we have corruption. Any time a wizard rolls a natural 1 on a spell check, bad things happen. Bad. Luck scores again come into play here. Lucky wizards may just have a spell misfire of some kind...but unlucky wizards may find themselves covered in boils, strange growths, or painful lesions. Not overnight, of course, but a lifetime of exposure to demons, extraplanar radiation, elemental energies, and the servants of Chaos isn't particularly good for one's health. Casting by an unlucky wizard makes it clear that there are consequences to dabbling with greater powers. Don't roll a 1 on the spell check!
So there you have the general summary. There are several levels of randomization, generally "bracketed" or "contained" by level and other factors, such that spells are never fully predictable. Magic in the DCC RPG really feels quite magical. When you cast a spell, you get a result associated with that spell, but the edges are "fuzzy" so to speak...