This is one of those strange articles that doesnt really get into hard mechanics, but mostly whimsy player-driven-fun. I've been dealing a lot with magic circles, having read the entire Dresden Files series in the span of a month, and one of many good effects its had on me is an appreciation for circular magic.
First of all, drawing circles is easy. Its so easy that Harry is able to explain it to someone who has never used magic before, on-the-fly: draw a circular shape on the ground (doesnt have to be perfect), invest some will into it and booyah. Done. With that in mind, accepting lots and lots of circles drawn out seemingly willy-nilly became much easier to digest, to the point that its one of those, "damnit I wish I'd read this before I started running my games," moments. Fortunately, seeing as hindsight is 20:20 its something I can learn from and take it into future adventures and campaigns.
How does this pertaint to D&D? Well, I noticed really quickly with pre-published D&D adventures that a common terrain feature are magic circles that impart some minor benefit when you were standing inside it. In Keep on the Shadowfell, for example, there's one outside the kobold lair (+1 to attack rolls), one in the graveyard (source of the undead), and one when you fight Kalarel (+2 to defenses and regen 5 for enemies only). Out of the three, only the one in the graveyard can be subjected to skill checks in order to fuck with it. I figure, why stop there?
I like to use terrain in my encounters: difficult terrain, pillars, tables, whatever I can think of that the players can fiddle with. I prefer such fiddling to take a single action in order to garner results, as there's nothing like a Skill Challenge in the middle of combat to make on player engage Mindless-Dice-Rolling Mode, and I think that magic circles are great candidates for this sort of thing.
For instance, in my Inquisitives campaign, the players run into a couple magic circles in the first encounter of the first adventure, when they are ambushed by (of all Heroic-tier humanoids) goblins. A goblin hexer has drawn a basic circle using rat shit and placed insect husks along the border. The circle is used to boost its area of effect and maintain its vexing cloud power, so that he doesnt need to burn minor actions to maintain it. The only drawback is that it must be centered on the circle. Characters can take it out by using a single action to do one of two things: break the circle or safely disperse it.
I call these one-hit circles "minion circles" since they can be "resolved" with a single dice roll. Basically, a character can opt to take some damage to break it (representing a magical backlash) or make a single Arcana check to safely disperse the magic channeled into it. The option to just break it and take damage lets characters with shitty Arcana modifiers let hit points do the talking. Either way, the effect is gone. In the case of this vex-circle, once its out the vexing cloud is also disrupted, so its win-win for the party.
You might rule that circles with nice benefits take a bit more effort to disable. A circle that controls or sustains a summoned demon, for example, might take two or more skill checks since it can make a big difference in the battle. In this case I would not only have a success count for itself, but also impart some other benefit to help them feel like they are contributing more (attack penalty, dazed for a round, slowed for a round, some damage, etc).
Now, if players want they can get a bit more creative. Another option is to allow Arcana checks to allow a character to alter the effect of the circle. For example, lets say they run into a circle that grants an attack bonus. Using Arcana or Religion or whatever, they could change it from a +1 power bonus to damage to a +2 power bonus to damage. Perhaps a +1 power bonus to one or more defenses. The effect would apply to everyone, though I could see a player making one or more Arcana checks in order to impart their will into the circle, affecting only those she wants to.
Another thing that a player could take a crack at is disrupting the circle, damaging all the creatures inside it. I would base the damage largely on what you would expect them to do with an encounter power, and might even roll on a push/prone/daze condition depending on how spectacularly the circle goes out (ie, with a bang).
Its one of those on the fly rulings that I would be really open to player creativity and DM flexibility. There arent any "hard" rules for this sort of thing, but I dont think its really prone to abuse since you as the DM get to decide how/if the circle can be augmented, as well as the DC and requisite skill. In most cases I would demand Arcana, but not demand training in the skill (since again, its super-easy to do). You could use Nature for fairy rings or stone circles, and Religion could apply to circles that work with healing, undead, etc. Depending on how common magic is, you could demand training, I suppose.
Some common circle effects:
- Power bonus to attack rolls.
- Power bonus to damage rolls.
- Power bonus to one Defense.
- Change damage from one type to another (for example, all attacks deal fire damage).
- Grant energy resistance.
- Grant regeneration.
- Grant regeneration only while bloodied.
- Grant one of more effects to specific targets only (determined by circle drawer).
- Sustain summoned monsters (likely undead, elementals, and/or immortals).