You are writing an adventure, and the plot just screams for an encounter with kobolds – then your start considering the players. These rough skinned critters make a perfect challenge for low level parties, but do they make sense for high level adventurers? Scaling your adventure is a tricky thing, but help has arrived.
Combat encounters have an encounter level (EL) associated with them. This EL is calculated from the challenge ratings (CR) of the combatants in that encounter. For example, an encounter with a Gorgon (CR 8) would be an EL 8 encounter. The calculations for determining the EL of an encounter are in g the Dungeon Master's Guide on page 48.
Scaling your adventure can be done in one or more ways. You can simple increase the number of monsters in the encounter, add class levels or a template, increase their hit dice or increase their size. The best thing to prepare yourself to scale your adventure is to read Chapter Four: Improving Monsters in the Monster Manual. It contains all the rules for advancing monsters and calculating the resulting challenge ratings.
Increasing the number of combatants is a good way to increase the difficulty of the encounter. It is simple and easy to calculate the resulting CR but should be used with care. If your encounter has too many monsters, it can really slow down the game. There is also a point where the increase in monsters can overwhelm a party regardless of the CR. If there are more monsters than party members, you may want to consider adding one to the CR or figure out a different advancement path.
Templates are also a way to give your monsters that little something extra. Templates are added to a base creature and give it access to special abilities as well as a host of other things. It is very important to make sure that the template you are trying to add to a monster is allowed. For example, the vampire template says it must be added to a humanoid or monstrous humanoid. Therefore you cannot add it to an animal or plant. As a final note on templates, be careful when you stack two or more templates on one creature. Sometimes they will have a synergy that makes them much more challenging than the CRs would indicate.
Adding a PC or NPC class level is a very efficient way to increase the challenge level. This method does have some things to watch out for. For example, some monsters cannot have class levels added to them. Only monsters that have “by class level” listed under advancement can gain levels. Another thing to pay attention to are monsters that have one or fewer hit dice lose that hit dice when you add class levels. For example, if you added a level of fighter to an Orc, it would just be a first level fighter (CR 1).
Adding hit dice or making the monster bigger is also a good way of increasing the difficulty of an encounter. Adding hit dice is very similar to adding class levels. When you add hit dice you also progress the number of feats and skill as well as the base attack and saving throws. How these increase depends on the type of monster you are advancing. Please refer to the Monster Manual (Chapter Four, again) for details on the various progressions. There is also a table in that same chapter that goes over the changes that take place when you increase a monster’s size. For every size category increase above large, the CR is increased by one. At some point, increasing hit dice also increases the size of the monster. The rule about adding one to the CR is ignored in this case.
Finally, the most overlooked way to increase the EL of an encounter is give a situational advantage to your monsters. For example, imagine the party walking in a steep canyon. Now add a couple of giants hurling rocks down on them from the top of the canyon. This makes the “land based” melee fighter almost useless. This would be an adjustment of at least +1 and probably +2. Anything that gives the bad guys an advantage should result in an increase in the encounter level. On the other hand, if you give the PCs an advantage the EL should be decreased.
All in all calculating the EL of an encounter is more art than science. The rules spelled out in the Dungeon Master's Guide and Monster Manual do a good job of trying to impose some order on the chaos. Using that number, as a starting point, a play test or two will help ferret out the real CR.