This column provides advice for DMs whose campaigns are in trouble. Do your players constantly bicker or complain about issues both inside and outside of the main campaign action? Do your best ideas fall flat? Have you set up a situation that you now wish you hadn't? Worry no more, because Jason Nelson-Brown has the answers to save your game!
Problem: Party Out of Bounds
The game that I DM has a large number of PCs -- about 12 (this always seems to be the number of players in my games). The average party level is about 8 (meaning I have some higher than 8th level and some lower than 8th level). Aside from 'just playtest it and figure out what works for your party' (which is what I ended up doing) is there a ballpark formula to determine what CR monsters I should be throwing at them? Figuring out the CR for a party of 4 is easy. I know that the specific magic items the PC's have influence the CR of a specific monster (and I've got that pretty much under control). I know (now) what works for my group, but I'm sure that there are others out there who are wary of large parties for just this reason.
--Joshua, from Ask Wizards
Your number of PCs is unusually high. I've run games that big, but it's certainly the exception rather than the rule. It brings up several different questions, because the dynamics can be very different if you have 12 people each running their own character versus having six people running two PCs each. Having players run more than one character is not ideal from a role-playing perspective, but it is a great insurance policy against low player turnout or against PC death -- you still have enough PCs to proceed even if only two or three players show up, and if a character dies the player is not stuck with nothing to do (if both of your characters bite it, it just ain't your night). Cohorts, followers, animal companions, a creature bound to service with planar ally or planar binding, or even creatures brought by a shorter-duration summoning or calling spell -- these are all 'extra characters' even if they aren't PCs. If these are a regular feature of your campaign, you need to account for that as a DM.
In addition to lots of players, you also have the complication of a spread of levels within the party. As a result, the variability in survivability of any individual party member is less predictable than it would be in a large party where everyone is at roughly the same power level.
The simplest answer to your question is no, there isn't a specific formula for determining what sorts of CR creatures to throw out there to challenge them. There are some simple tips for dealing with large parties, though, and how to challenge them.
1. One Plus One Does Not Equal Two
The simplest expression of this rule is that you can't just sum up party levels and assume equivalence. Twenty 4th-level characters are in no way equivalent to four 20th level characters. That's obvious. Are 12 8th-level characters equivalent to eight 12th-level characters? This is a much closer comparison, but still the answer is no, and the reason lies in the intersection between frequency and effect.
Your lower level group gets more opportunities to do things because of their large numbers, but the things they do are of lesser effect -- although this is not an absolute lesser. It is a relatively lesser effect that magnifies the impotence of the attack against more powerful enemies. When your 8th level wizard casts fireball at a group of typical 4 HD monsters, most of them will fail their save and be toasted. Against typical 8 HD monsters, a fair number will save, and even those who fail will only be modestly injured (an 8 HD monster doesn't just have twice the hit points of a 4 HD monster -- often it's closer to three times as many). Some creatures will simply avoid many or all of the effects because of spell resistance, fire resistance, evasion, or a similar ability. It is unlikely that a creature that is the equivalent of a 12th-level character will be substantially affected by the fireball spell. Almost all creatures at that level will have energy resistance, SR, or both, plus very good saving throws. Even in the best-case scenario (no resistances, fails the save), the impact will be negligible, because the damage represents only a small slice of the target's hit points.
But in a party of 12 people, you might have three wizards or sorcerers all casting fireball, so they'll do 24d6 damage, right? No. They'll do 8d6 damage three times, which is entirely different. Against a target with fire resistance 30, an average 24d6 fire attack would still do 54 points of damage -- three average 8d6 fireballs would do nothing. Your fighters in the big party may get a bunch of attacks on the boss monster, but if few attacks land and fewer still get through the special defenses of a high-level monster, what's the point?
In the other direction, a creature with a mega-damage spell or special attack may wipe out some characters (especially your lower-level types), even if they make successful saves. These monsters' attacks will almost always hit, their damage will be hefty enough to tear through most resistances possessed by lower level PCs, they have enough of an attack bonus that they can afford to Power Attack for increased damage, and they will have more special attacks that your characters are less likely to resist or perhaps survive.
This is a long way of stating, don't just amp up the CR of the boss monsters, because things can get out of hand quickly when the power differential breaks. It is OK for you to kick up the CR by a few notches to beef up encounters, but when the individual CRs for enemies start getting to four or more higher than the average party level, you are getting into very dicey territory. The likelihood of the entire party getting wiped out may not be that much different, because sheer numbers dictate that some should get away, but the incidence of individual fatalities goes through the roof when you pit the party against monsters with too-high CRs.
2. More is Better
The simplest method for dealing with large numbers of PCs is to use large numbers of monsters. Since you have two to three times as many PCs as a 'normal' group, increase the number of enemies without adjusting the CR. Instead of a pair of minotaurs for four PCs, the big party is faced with half a dozen -- but the ratio is still one minotaur per two PCs. This way, you increase the total mass of enemies without using any individual enemies that are going to overpower the PCs. The monsters are still at a commensurate level with what the individual PCs can do. This has the side benefit of allowing PCs (and the monsters) to use more tactics such as flanking moves, aiding another, or tripping or bull-rushing for the benefit of their allies. It makes feats such as Great Cleave and Whirlwind Attack more useful. It also balances the effects of ray spells and other effects that are extra-powerful against one target. If you tend to face only one or two enemies at a time, these spells become comparatively more powerful than multi-target or area spells -- having to deal with multiple targets underscores their limitations and encourages players to be more versatile in their spell selection to deal with a variety of combat conditions.
The problem is that this only works with monsters that are supposed to be in groups -- the minions and hordes of evil. If you have in mind one signature villain as the focus of the forces of evil, you can't really just have three Big Bad Guys. That sort of ruins the effect. As long as there is only one Big Bad Guy, the party can concentrate its firepower and overwhelm him with numbers.
The trick, then, is to make sure that Big Bad Guys have Big Bad Henchmen. Hey, the Emperor had Darth Vader (and Maul and Tyrannus), Cobra Commander had Deastro, Dragon Highlord Ariakas had Kitiara and Lord Soth … the list goes on. There is clearly an Evil Leader, but the Evil Leader has a couple of associates who are not as tough as she is but who are tough enough that they can't be ignored, preventing the party from ganging up on the super-villain. As a general rule, the party has greater ability to concentrate its firepower and focus its powers to eliminate single targets than the bad guys do, at least if the players know what they're doing. You must diffuse that firepower by providing extra targets. This is one reason that summoning spells and minion spells such as planar ally or elemental swarm are good for your chief villains to use -- their minions suck up attacks and damage that would otherwise hit the villain. If you're busy dealing with the Elemental Horsemen of the Apocalypse, the evil St. Lareth the Beautiful can finish his evil ritual (or concentrate his attacks to wipe out one of the party members).
Dealing with a large party is not easy, because it requires a different scale of gaming. Stealth is out the window with so many PCs unless you have lots of magic or break up and subdivide the party frequently so that specialist groups explore separate parts of the adventure. Balancing combat is also hard, because a big party has the ability to overwhelm smaller numbers of powerful foes -- a pack of adventurers taking down a dragon or lich like a school of piranha! A large party is also subject to total wipeout if an enemy is far enough above the PCs in power, so that their abilities almost always work and almost always hurt, while the PCs need luck to cause much damage in return.
The best course is to keep CRs the same or maybe 1 higher but increase numbers equivalent to the increase in the party's numbers (vs. an assumed, typical four-person party). Or you could keep numbers the same but make the monsters a bit tougher, say a 2 or 3 CR increase. Creatures 4 or more CR above party level are possible, but that's when you need to be very careful about how the monster and the combat environment are going to affect the battle.
Lastly, never leave a boss monster alone against a big party. Surround her with minions, summoned or magically bound monsters, and/or some tough henchmen to help diffuse party power and prevent them from ganging up on the leader.
About the Author
Jason Nelson-Brown lives in Seattle with his wife Kelle, daughters Meshia and Indigo, son Allen, and dog Bear. He is an active and committed born-again Christian who began playing D&D in 1981 and currently runs one weekly campaign while playing intermittently in two others.