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What I need for d20 creature design:
A software program where you plug in the stats of a monster and it runs a few tests (expected damage per attack vs various character types and things of that sort) and tells you what the suggested CR is. I would be perfectly happy with it flagging things it couldn’t factor in, but I wantsss it.
Also, why are there CR 1/2, CR 1/3, and CR 1/4 creatures, but no CR 2/3 or CR 3/4 creatures? It seems like the lack of resolution between CR 1/2 and CR 1 hurts low-level encounter writing.
It’s because fractional and integral CRs work differently.
A CR of ¼ means that "four of these guys make up an EL1 encounter". A CR¾ encounter, by that token, would mean "four of these guys make up an EL3 encounter".
But if you have an EL3 encounter with four equal-CR creatures, each of them would actually be CR½—two CR½ creatures equals EL1, and if you double the number of creatures again, you add two to the EL, ending up with EL3.
If you want two unequal combatants in an EL1 encounter, I’d suggest maybe using two CR¼ creatures, but giving one of them a level in a non-associated class (see MM 3.5, p294).
As for software, I haven’t found anything with the level of detail you require (although you might want to pick up Expeditious Retreat Press’ Beast Builder, if you don’t have it already).
I can, however, recommend Pen, Paper & Pixel’s d20 Encounter Calculator for working out ELs from wildly differing CRs.
A CR of ¼ means that “four of these guys make up an EL1 encounter”. A CR¾ encounter, by that token, would mean “four of these guys make up an EL3 encounter”.
Right. I know that… and I think CR 3/4 would be useful. Sometimes you want variation in low-level encounters. What if I want an EL3 encounter with three equal guys?
I use the d20 Encounter Calculator all the time. It is great.
I’l look into Beast Builder. Right now, I mostly just eyeball things in comparison to the standard beasties.
I see what you mean about the fractional CRs. For three equal guys for an EL3 encounter, working backwards, one-and-a-half guys would be EL1, and one guy would have a notional CR of 2/3.
Since (as you point out), there are no CR 2/3 creatures, you could possibly make them by taking a CR 1/6 creature (there are four in the MM: donkeys, lizards, monkeys and ravens), giving them a level in warrior and fudging their stats a bit.
For the purposes of this exercise, we’ll discard the donkey as too bizarre ("Dude, we got our asses kicked by… asses."); the others should probably get an extra four points of Int, so they can use their class abilites. It shouldn’t affect their CRs appreciably.
(In this case, when I say "class abilites" I mean equipment. Since, as a rule of thumb, a third of NPC CR comes from equipment, if a base CR 1/6 creature with a level of Warrior lacks equipment, it becomes a CR½ creature: 1/6 + (½ × 2/3) = ½.)
If you go for a straight level add, then they get an extra 1d8 hp, +1 BAB, +2 Fort save, 4 skill points, a feat and 3d4×10gp worth of equipment. (It’s really too much to ask to give them any more treasure.)
This works okay for the monkey and (with a bit of a stretch) the raven, but what about the lizard? It’s a quadruped, after all.
If you have Green Ronin’s Advanced Bestiary, you can slap the Bipedal template (a CR-neutral mod) on top of that, so you get an EL3 encounter that looks something like this:
Lizard Pygmy War1 (3): CR 2/3; Tiny Humanoid (augmented animal); HD ½d8 + 1d8; hp 7 each; Init +2; Speed 20 ft. (4 squares), climb 20ft.; AC 16 (+2 size, +2 armor, +2 Dex), touch 14, flat-footed 14; Atk bite +5 melee (1d4-4), dagger +3 melee (1d2-4) or shortbow +3 ranged (1d3-4); SA none; SQ low-light vision; AL Neutral; SV Fort +4, Ref +4, Will +1; Str 3, Dex 15, Con 10, Int 5, Wis 12, Cha 2.
Skills and Feats: Balance +10, Climb +14, Hide +12, Listen +3, Move Silently +4, Spot +3, Survival +2; Stealthy, Weapon Finesse.
Equipment: Tiny leather armor, tiny dagger, tiny shortbow (20 arrows each), smokestick, 12gp each in miscellaneous items and treasure.
Although their AC, hp and chances to hit might be better than some CR1 creatures, they only do a single point of damage with each successful attack, nor have they special attacks. Str, Cha and Int (even when boosted) are particularly low, too.