How to design good encounters? (DM v.s. PC)

[RoboEmperor]

It's my turn to DM a campaign, and the only way that will remotely interest me is if it's a DM vs PC game.

So the concept is that in every fight where everyone rolls average rolls, I will lose. I can only kill the PCs if and only if:
1. They get really unlucky with their rolls.
2. I get really lucky with my rolls.
3. They make a few mistakes. My goal is two. Two mistakes and none on my part will result in a death or two.

So since this will be kind of my first time DMing a big campaign, I need some tips on setting up the encounters. So how would I design the encounters to fit the above criteria?

Party size is 4.

So I know that CR 6 = a monster that needs a party of 6th level characters to bring down, but what about two CR 6 guys? How about 3? I know CR is broken, but I need some general guidelines to start with.

I don't plan on doing any "counters", like if the PCs are all spellcasters I won't go full anti-caster equipped creatures, or if the PCs are all using sonic weapons I won't bring out anti-sonic gear, etc. If party abuses invisibility, I won't have every grunt carrying dusts of appearances. Only returning grunts will bring flour and such.

[DarkSoul]

Start by reading the DMG. It answers most of your questions.

The short version:

CR is designed around a 4 person group.

An encounter of a given CR should require a group of that level to expend about 25% of their resources to overcome. A CR 6 encounter is a reasonable but not overly difficult encounter for a 6th-level group. Your encounters can exceed the party's level by up to 4, but that's a very deadly encounter and should be saved for major campaign-related battles. A party level +2 or +3 should probably be the only encounter the group faces that day.

Except at very low CRs, doubling the number of creatures in an encounter raises its EL by 2. 2 CR 6 is an EL 8. 3 CR 6 is an EL 9. For mixed CRs, a CR X and a CR (X-2) together will create an encounter of EL (X+1). A CR 7 with a CR 5 is an EL 8.

Quick aside about CR vs. EL. CR is the difficulty of a single element (monster, trap, etc.) of an encounter, and EL is the final total of all the elements. A CR 6 monster fought around a CR 6 trap is the same EL as 2 CR 6 monsters.

[Nuada99]

An encounter of a given CR should require a group of that level to expend about 25% of their resources to overcome.

In my opinion, it should be more like 33%. Three fights a day is what the GM should strive to have the party wanting to do, and if a 4th (or 5th fight, in truly harsh circumstances) happens to come along, it should be a real problem, since all of their "alpha strike" capabilities are probably burned up.

Of course, this doesn't mean that the encounter only deals a net of 33% of the characters' hit points. Healing is highly efficient. Even at low levels, an encounter should probably deal somewhere around 50% of the party's total hit points in damage, under normal circumstances. At higher levels, this can be as much as 100%, just because healing (and other tricks like miss chance, regen, etc.) contribute so much to the fight.

Some other theory, since we're talking about it:

- Monsters should hit players about 35-40% of the time, assuming there isn't just one monster on the field. If it's just one monster on the field, it should probably have a way to make at least one attack per round per player character, and ideally more. The damage should also be spread around by these attacks, to prevent the "dogpile all the damage on one character and inevitably kill them" scenario. There are times to do this, of course, but you should be reasonably sure the party is capable of propping up a single character who is taking a relentless barrage of damage like that.

- Players should hit monsters around 65% of the time, under normal circumstances. Monsters might have an ability that temporarily makes this much harder, but in general, wasting your turn on a miss, and then having to wait 30-45 minutes for your turn to come back around isn't much fun. However, missing should still happen, and players should eagerly plead for buffs from the buff classes to improve their average to-hits. That +2 to hit from Good Hope should be meaningful enough for someone to have wasted a standard action to cast it.

- Fights, on average, should last about 3 rounds. This gives everyone a chance to do some stuff, including the monster, that demonstrates just who they are and how they operate. Some fights will go longer, and some will go shorter, but in general, 3 rounds is the sweet spot. After the 3rd round, the above suggestions about damage per encounter should start to change, to the party's detriment. Fights which last longer than 3 rounds should make the party burn more of their resources. The monsters get more swings against the party, so they deal more damage. Casters and other classes with "usable x rounds per day" abilities (bard, barbarian, etc.) should start worrying about whether they should keep burning the candle at full flame, or maybe ease back so they're not tapped out for the next fight.

- As a result of the "fights should last 3 rounds" idea, monsters need enough hit points and/or defenses (ER, DR, etc.) to actually survive against the party's normal damage for 3 rounds, and ideally, a little bit more than that. Fudge factors are important because players will ALWAYS surprise you.

Remember that you only have 1 brain to oppose all the plotting and conniving of all of your players' brains. You will never have the upper hand in terms of clever ideas (though you might occasionally get one past them, it will be far more common for them to get one past you.)

Also remember that fights are only a way to further your plot, and keep players engaged. Good fights have consequences, ranging from "you got an interesting clue" to "now the local constabulary is after you" or "now the Dyspartium of Ulchor knows your names". Fights are one of the easiest ways for each character to feel like they're adding something to the story. "And then the monk kicks off the guy's shoulder, flipping backwards to land an axe kick on the top of the orc's skull. The orc crumbles like his strings were cut." Oftentimes, outside of combat, the combat-heavy classes have far fewer opportunities to directly contribute to the story's outcome.

Another fun trick to let the character shine during combat is to add moral choices to a few of them. Civilians in danger add a non-combat element to a fight, and might cause one or two characters to spend a round helping them instead of dealing damage. This is a big deal, both from the story perspective but also from the crunch perspective of how challenging the fight is. Setting a fight inside a burning building where the party needs to rescue a scroll hidden inside the (wooden) chest in the back room is an exciting fight. It doesn't always have to be about beating the bad guys. Sometimes it's just about foiling their plan or achieving an objective that happens to be more difficult because bad guys are in the way.

For villains, fights should almost never be set up where the only objective is to beat the villain. If you do this, you are committing to either a dead villain or a dead party. (Of course, eventually, this should happen, but ideally, the party should have to set it up themselves; if the villain gets the choose the fight, the fight should always be about some other objective, like "save the mayor from the <whatever>, allowing the villain to steal the <other whatever> unimpeded.")

Let your players do cool things. I can't stress this enough. When you start a fight, describe the environment, and make sure there are odd bits of scenery that a clever player can interact with. If you're in a theatre, the rogue can assume there's curtains he could use to swing down from the balcony to the stage. In a nobleman's house, there's stone banisters on the stairways which one can use to slide down.

Also try to avoid the "both sides step up to each other and use full attacks every round until one side wins" stagnant fight. Fights with movement are always better than fights without it. Monsters with ways to either move without giving up their damage potential, or ways to move the characters around (knockback attacks, etc.) really change the dynamic of the fight. Obviously, like many of these suggestions, this shouldn't be used in every fight, but it's a useful way to change things up.

It's also good practice to be excited about your fights, and over-react (in a positive way) to how cool the players are being during your fights. Reinforce cinematic behavior, and help the players visualize just how awesome it is when they pull off something truly savage. Everyone can sit at the table and say "I rolled a 25 vs AC, and deal 21 points of damage with my great axe". But that's pretty dull, especially the 100th time it happens. Instead, when that 21 points kills the bad guy in question, describe it in excited tones, "the axe swings through the air, trailing a line of blood as you cleave through the ghoul's torso." Encourage your players to describe things as well. It really makes everything more cinematic.

In the end, it's all about having fun. Everyone is at the game to have a good time hanging out with his/her friends. They want a chance to hear a good plot, employ clever ideas to foil said plot, talk to the barkeep's sexy daughter (or son; no one's judging), maybe punch a camel or two, and generally feel like a total badass hero. As a GM, you should help them achieve this, but remember that heroes are always defined by their villain, and by how steadfastly they get back up after being knocked down. Don't be afraid to truly abuse the crap out of the heroes, as long as you celebrate their tenacity when they overcome the trial you gave them.

Have fun and good luck!