Pretty cool, huh? I had fun doing it besides! I made a mistkae, actually, this would be a CR7 creature. I forgot to add a +1 for the increase in size, and the +1 for using the Elite Array. I'll explain below.
Okay, here's the break down, all of which are explained in the SRS for 3.5. Here's a link to the exact page I was refering to:
HERE
First was this section:
Increased Hit Dice: Intelligent creatures that are not humanoid in shape, and nonintelligent monsters, can advance by increasing their Hit Dice. Creatures with increased Hit Dice are usually superior specimens of their race, bigger and more powerful than their run-of-the-mill fellows.
Next, the abilities (four factors play here):
First:
Elite Array: The elite array is 15, 14, 13, 12, 10, 8. While the monster has one weakness compared to a typical member of its race, it is significantly better overall. The elite array is most appropriate for monsters who add levels in a player character class.
Second:
SIZE INCREASES
A creature may become larger when its Hit Dice are increased (the new size is noted parenthetically in the monster’s Advancement entry).
Worg: Advancement: 5–6 HD (Medium); 7–12 HD (Large)
Granted, it lists a max of 12 hd, so I may and probably should lower it by one (from 13 to 12), and to do so would lower it's CR to 6. That's explained momentarily.
A size increase affects any special ability the creature has that is affected by size. Increased size also affects a creature’s ability scores, AC, attack bonuses, and damage values as indicated on the tables below.
Table: Changes to Statistics by Size
Medium to Large- Str+8, Dex–2, Con+4, Natural Armor+2, AC/Attack–1
Table: Increased Damage By Size
Old damage: 1d6 New damage: 1d8
Third:
Racial Bonuses
A worgs initial stats are: Str 17, Dex 15, Con 15, Int 6, Wis 14, Cha 10
Which breaks down to racial bonuses of: Str+6 Dex+4 Con+4 Int-4 Wis+2 Chr +0
Lastly, advancement bonuses:
Ability Score Improvement: Treat monster Hit Dice the same as character level for determining ability score increases. This only applies to Hit Dice increases, monsters do not gain ability score increases for levels they “already reached” with their racial Hit Dice, since these adjustments are included in their basic ability scores.
Thus, with 9 more hd, he would get an bonus of +2 to abilities, where the creator chooses (one for ever 4 HD).
Third, CR determination:
Worgs base CR: 2
When you improve a monster by adding Hit Dice, use Table: Improved Monster CR Increase to determine the effect on the creature’s CR. Keep in mind that many monsters that advance by adding Hit Dice also increase in size. Do not stack this CR increase with any increase from class levels. In general, once you’ve doubled a creature’s CR, you should closely watch any additional increases in its abilities. Adding Hit Dice to a creature improves several of its abilities, and radical increases might not follow this progression indefinitely. Compare the monster’s improved attack bonus, saving throw bonuses, and any DCs of its special abilities from the HD increase to typical characters of the appropriate level and adjust the CR accordingly.
Table: Improved Monster CR Increase
Animal, magical beast, monstrous humanoid +1 per 3 HD added
Size increased to Large or larger +1 to CR
Monster’s ability scores based on elite array +1 to CR
So, with only 8 added instead of 9 that would be a plus 2CR instead of a plus 3. That and the other two CR increases brings brings the Worg to CR6.
Other improvements associated with HD increas:
INCREASING HIT DICE
As its Hit Dice increase, a creature’s attack bonuses and saving throw modifiers might improve. It gains more feats and skills, depending on its type, as shown on Table: Creature Improvement by Type.
Note that if a creature acquires a character class, it improves according to its class, not its type.
Table: Creature Improvement by Type
Magical beast Hit Die:
d10 HD Attack Bonus:
(as fighter) Good Saving Throws:
Fort, Ref Skill Points:
2 + Int mod per HD
