(and Confusion About XP, CR, EL, and ECL)
3E has some very useful tools for creating and running encounters but I've noticed a fair number of people getting confused in how to use them, especially Encounter Level (EL) calculations. I think the two-letter abbreviations alone add to some of the confusion, but the DMG section on EL is not exactly crystalline in clarity and probably accounts for most of it. First some definitions:
Unfortunately, encounter design is still not an exact science. The more complicated an assortment of monsters you use the more difficult it is to know how it will stack up against the PC's. The EL chart has limitations. It doesn't hold up well when you are using a very high number of opponents or when the PC's are widely spread in levels. In those cases the subsequent encounters may play out very differently than their EL might indicate. The system is also based on 4 character of equal level, and of standard levels of equipment and ability. If your PC party has a lot more than 4 characters (or significantly fewer), or if those characters have way too much equipment value, or non-standard special abilities, then the system CANNOT account for it - YOU have to. When the basic assumptions of the system don't hold true it can be difficult to take the deviations into account and in extreme cases even attempting to do so may be futile.
When you plan an encounter the simplest, most direct way is to just compare the CR of a monster to the average level of the party. A CR 5 monster is a standard challenge for a party of four 5th level characters. If your party is a few more or less than 4 characters it shouldn't be a problem. That's because you're still dividing the total number of levels in the party by 4 to find the average and a little more or less still works just as it should - you're finding an average after all. So long as there aren't wild differences in levels or the number of characters it'll work as it should.
The EL chart is for when you're going to be using more than one monster of the same type. It simply tells you how many monsters of a given CR will make an appropriate challenge for the party you're dealing with. For a 10th level party a single CR 10 monster is the benchmark, but if you want to have four monsters then the CR for those monsters should only be 6. Or to put it another way, if the monsters you want to use against a 10th level party are CR 6 then you'll need four of them in order to provide the basic level of challenge.
Now as long as the monsters you're using are equal in CR then the chart works fine and it doesn't matter a lot if the monsters are of widely different types - just so long as their CR's are equivalent. It's when you want to use monsters of widely differing CR's in the same encounter that the chart starts to get fuzzy. The couple of paragraphs on "matched pairs" and "mixed pairs" will tell you (confusingly) how things compare, but doesn't provide a simple formula that you can just plug monsters into and have it spit out the EL. When you make encounters complex in that way you're already getting beyond what a simple formula can do for you anyway. When you want or need to have a complex variety of CRs among your opponents you simply have to do your best to find equivalency.
In other words if you have three different kinds of creatures, each of a different CR then pick one of the creatures as a basis to work from. The highest CR is probably best. This allows you to break down multiples of the lower CR creatures as a "partial" equivalent to the higher CR. It just seems easier to do it that way than building up multiples and trying to figure out if they are equivalent. It also gives you a clear indication up front of what the toughest thing in the encounter will be and that everything else about the encounter should be easier to handle.
For an example, take an 8th level party for whom you want to create a tough encounter using three different CR creatures. Assume that there will be one CR 8 creature and several other lesser CR creatures. From the section on "Matched Pairs" we know that two creatures of an identical CR make an encounter that is effectively two CR higher. (Remember the CR itself doesn't change, we're just seeing what we can use and in what quantities.) So two CR 8 creatures would be equivalent to an encounter with a single CR 11 creature - a reasonably tough encounter. But we only want to use one CR 8 creature and several lesser ones. One of those CR 8 creatures we can then break up for "partial" creatures of lower CR's. Again, using the Matched Pair guidelines two CR 6 creatures are equivalent to a single CR 8 encounter. We can then subdivide again getting two CR 4 creatures for one of the CR 6 equivalents. That would leave us with one CR 8 creature, one CR 6 creature, and two CR 4 creatures that would constitute an EL of 11.
Something that the EL chart doesn't show you, and that the DMG doesn't remind you about, is that those two CR 4 creatures in that encounter are a VERY low challenge to an 8th level party. As part of this larger group of monsters they help to make things more challenging but they themselves are not much of a threat and thus are not likely to figure prominently in the fight. The encounters I see people stumbling over when trying to figure the EL for them, are of this type - where there are low CR creatures that just aren't going to figure prominently in the fight. This is something you need to keep aware of as you design encounters. You need to decide if those weak creatures are really adding anything worthwhile to your encounter
Lets try another example using the Mixed Pair guidelines. We again have an 8th level party. We want the encounter to be of standard difficulty but the monster we want to use is CR 7 which is a bit lower. To bump it up the Mixed Pair rule says we can add an additional creature whose CR is three less, a CR 4 creature, and the EL for this mixed pair then becomes EL 8. If we wanted a tougher encounter than that we'd start with a CR8 creature, add another creature of CR 5 and get an EL of 9 - one higher than just the CR 8 creature would be by itself.
This should then illuminate how the EL chart gets used - it isn't just a plug-and-play formula spread out in a chart. When you design encounters you need to design encounters. You need to consider what level the party is, what degree of challenge you want to present, what kind of creatures you want to use and then mix, match, and adjust circumstances until the encounter fits your needs. Having a large variety of creature types and CR's is not really going to contribute to your encounters, it just makes them harder to set up and run. Which is not to say that bland, repetitious fights of 4 PC's against one monster are good either, but the best encounters are those that make the best use of what's there, not those that simply have a lot of potential that goes unused. An encounter with 6 goblins, 2 ogres, a dire lion and a sorcerer is going to be difficult to gauge the EL, but a well-run encounter with 5 ogres and an Ogre Mage is easier to gauge and likely to be much easier to run and more memorable to the players. In a fight against a lich, a beholder, and 4 giant rats nobody will care what the giant rats do. For the rats to be meaningful to the encounter in any way they would have to have some special purpose or ability beyond absorbing a sword blow. Without that they just don't affect the EL.
I mentioned above about adjusting circumstances which is a VERY useful tool. If you have specific monsters that you want to use but the EL will be too high for your PC's then either give the PC's some circumstantial advantage or a disadvantage to the monsters. Likewise if the EL will be too low but you don't want to add more or tougher monsters then give the monsters some circumstantial advantage or give a disadvantage to the PC's. Special added attacks, immunities to expected attack forms, restricted movement, and outright bonuses or penalties to attack rolls for some reason are all good candidates for adjusting the EL of an encounter in the direction you want and will often make it more interesting and memorable.
One last word about XP and that is the use of the XP distribution method from the FRCS (Forgotten Realms Campaign Setting book) that is now used by 3.5 instead of the original system. It is a slightly more complex method that was originally intended just to deal with a situation of a high level character working directly with lower level characters. The FRCS method keeps down the amount of experience that the high level character gets because it compares each character individually to the challenges faced rather than the PC's as a group. I'm not sure that it's a necessary change and that you might be better served by continuing to use the original 3.0 method.
Reasons to use the new method would be if you're going to have a high level NPC traveling with the party, if the PC's have a lot of followers and hirelings of much lower level than themselves working with the party on adventures, or if the current PC roster is going to rotate often enough to routinely see differences in levels, then the 3.5/FRCS method gives a better spread for XP distribution (even if it does take a tad longer to total things up at the end of the night). If your campaign isn't dealing with those kind of circumstances and the PC's almost always work together on every adventure without ever becoming higher or lower level than another PC, then you might save yourself the hassle and use the original DMG method.
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