What happens to CR if I change the average ability scores?

[Tonguez]

Okay Tila is an abused and immaciated Ogre slave. Her years of neglect and beatings have left her retarded (even by Ogre standards) and weak
I've reduced all of Tilas abilities by -5 (except Int which is reduced -2 otherwise she would have only had Int 1))

Tila Ogre slave
Str 16 , Dex 3, Con 10, Int 4, Wis 5, Cha 2

So what is her CR?

Another one

an intelligent Troll with Int 12 (+6) and Wis 15 (+6) what is its CR?

 

[da chicken]

I'd say CR 1/2. Ogres start at 2, but she's worse in combat than an Orc (CR 1/2). Only reason she isn't CR 1/3 or less is because she has 4 hit dice.

Troll: No significant change, IMO. Maybe CR +1. Thing is, a troll is not going to benefit much from higher Wis and Int unless they start gaining class levels (in say, Ranger).

 

[Elder-Basilisk]

Officially changing stats does not change the CR of a monster. Unofficially this is the realm of guestimating and playtesting.

Your powered down ogre is definitely less threatening than a standard 21 strength ogre. However, she's still significantly more threatening than a similarly equipped orc (natural armor makes up for the dex penalty to AC, she's still stronger, just as tough, and still has reach and the ability to wield huge weapons). I'd give her a CR of 1 to 2 depending upon her equipment (the CR 2 possibility is based on comparison to a 2nd level fighter not to an ordinary ogre--each CR covers a range of challenges).

Your smarter and wiser troll wouldn't get a CR boost from me either. Unless he has a class, all he gets out of his wisdom is a better will save. (And if trolls are going down to mind-effecting spells, then they're probably not real challenges for the party considering that all the lethal mind effecting spells that effect a troll are level 4+). His increased scores, however, do mean that he will most likely be encountered with allies, traps, or in a favorable circumstance. All of these increase EL (and the first two have their own CRs as well).

 

[da chicken]

Officially changing stats does not change the CR of a monster.

He's right about that. What I normally do is apply a +/- ~10% for hard and easy encounters. The method is intended for single encounters that are more difficult, not for creatures that are, but it provides a good basis for the ad hoc you're looking for.

For example, skeletons (normally CR 1/3) gain significant benefits for being created and fighting in a desecrated area. Six skeletons is normally about EL 2, but a desecrate on a nearby altar (+2 hp, attack, damage, saves, -3 to turn checks) would probably make it EL 3 (maybe 4). I'd grant +10% XP for this encounter due to the difficult circumstances involved.

That's probably what I'd do with your Ogre. -10% XP for her. If the Troll manages to use his Int & Wis to very effect, then I might grant +5% XP.

 

[Xarlen]

I think effecting the critter's stats, to a reasonable degree (+8 all together) shouldn't enhance it all that much. Remember, the monsters in the MM are 'average', just like the average human has 10s and 11s in all stats.

Thus, a critter that's a little more advanced then it's fellows isn't a problem. I plan on having my group clash with a Green dragon, and her intelligence is going to get raised, simply because she's a mastermind, and definetly should be smarter then the party mage. She's a friggin' DRAGON.