The following rules allow you to adjust monsters, increasing (or even decreasing) their statistics and abilities while still creating a balanced and fun encounter.
Adding racial Hit Dice to a monster is a similar process to building a monster from scratch. As additional Hit Dice are added, other abilities increase in power as well. Additional Hit Dice usually results in better attack bonuses, saves, hit points, and skills, as well as more feats. It can also include additional spellcasting capability and other powers.
When advancing a monster by adding racial HD, you should start by deciding what you want the monster to become. In most cases, this means merely a tougher, stronger version of an existing monster. Note the desired CR of the new monster. This is also the point at which you should decide whether the creature is going to increase in size. As a general rule, creatures whose Hit Dice increase by 50% or more should also increase in size, but GMs should feel free to ignore this rule if warranted by the individual creature or situation.
Next, determine how many Hit Points the base monster receives per Hit Die (see Table: Average Die Results for average results based on the die type). Using Table: Monster Advancement, add up all of the values in the Hit Point Change column for each increase using the Higher CR column. For example, if the base monster was CR 3 and the new monster is set to be CR 5, the total would be 25 hit points. Next, add additional Hit Dice to the monster to increase its hit points by the desired amount. Note that if the creature increases in size, its Constitution may also increase, as noted on Table: Size Changes, granting it additional hit points that might offset the need for additional Hit Dice (this also applies to any other Constitution increases).
These values are not absolute. Some monsters have fewer hit points than normal for a creature of their CR and rely on a higher AC or other defenses. Some creatures are primarily spellcasters and typically have fewer Hit Dice. When advancing your monster in this way, be sure to take these factors into account and adjust your monster accordingly.
Once you have determined the number of additional Hit Dice possessed by the creature, use this number to modify its other statistics. Start with ability scores. For every 4 additional Hit Dice gained by the monster, add 1 to one of its ability scores. In addition, make any modifications to its ability scores based on an increase in size, as noted on Table: Size Changes.
When adding skills, check to see if the creature's Int modifier changed. If it is unchanged, simply multiply the total number of ranks per Hit Dice gained by a monster of its type times the total number of added Hit Dice and add that number of ranks to its existing skills. If its Intelligence modifier has increased, perform the same calculation as if it had not increased and then multiply the change in its Intelligence modifier times its new total number of Hit Dice and add that number of additional ranks as well (adding new skills as needed to spend all of the ranks). If the creature changed size, make sure to adjust its Fly skill and Stealth skill bonuses (if any) as noted on Table: Size Bonuses and Penalties.
Next, give the creature additional feats. Creatures gain one feat at 1 Hit Die and one additional feat for every 2 Hit Dice above 1. Most additional feats should be focused on increasing the creature's combat abilities, but metamagic feats and skill feats are also possible choices depending on the creature's role.
Next, adjust the creature's derived statistics, such as its initiative, AC, saving throws, melee and ranged attack bonuses, BAB, CMB, and CMD. Adjust any special attacks or qualities that are based on the creature's size, Hit Dice, or ability scores. If the creature changed in size, be sure to adjust its AC, attack, CMB, and CMD accordingly (as noted on Table: Size Bonuses and Penalties). Table: Monster Advancement also tracks the average change to the creature's AC, attack rolls, and damage rolls. Add up these values for each step of change between the creature's original and new CR. If the creature changed size, make sure to make changes to its natural armor bonus, as noted on Table: Size Changes. If the creature does not meet these averages, you should consider adjusting its ability scores or Hit Dice to get it closer to the target.
Finally, compare the new monster's statistics to those presented on Table: Monster Statistics by CR for a creature of its adjusted CR. Note that if the original creature deviated from these values, the new one should do so in a similar fashion. For example, if the original creature had higher than normal hit points but a lower than normal AC, the creature should maintain that balance at a higher CR (even though its hit points and AC both increased).
Of all the methods of advancing a monster, adding class levels requires the most adjudication and careful comparison. Some classes truly add to the power and abilities of some monster types, while others do not. For example, adding levels of barbarian to a hill giant can be a great addition, whereas adding levels of sorcerer is less useful. When adding class levels to a creature, take the following three steps.
When adding class levels to a creature, the first step is to determine what role the base creature fulfills. There are three basic roles into which a creature might fall. A creature can fall into more than one role if its abilities are diverse.
Combat: This creature is designed to be good at melee or ranged combat with a weapon or its natural weapons. In either case, these monsters have a number of feats and abilities to enhance their combat prowess (or are good simply by nature of their Hit Dice and ability scores). If a creature does not possess many spells, special abilities, or skills, it is a combat monster.
Spell: Spell creatures possess a large number of spells that allow them to attack or harass their enemies. These creatures usually have lower hit points and relatively weak attacks as compared to the averages for creatures of their CR. Note that creatures that only possess spell-like abilities do not fall into this role, and are usually considered combat or special.
Skill: Creatures of this type rely on skills (usually Stealth) to ambush or take down their prey. This also includes creatures who take advantage of the environment or spells, such as fog or invisibility.
Special: Creatures that do not fall into any of the other categories usually rely on special abilities and powers to attack their foes. They might be tough or dangerous in physical combat, but the threat is greatly increased by their special abilities.
See monster roles for determining key classes.
Once you have determined the creature's role, it's time to add class levels. The first step of this process is to modify the creature's ability scores. Creatures with class levels receive +4, +4, +2, +2, +0, and –2 adjustments to their ability scores, assigned in a manner that enhances their class abilities. Creatures with NPC class levels do not receive adjustments to their ability scores.
Next, add the class levels to the monster, making all of the necessary additions to its HD, hit points, BAB, CMB, CMD, feats, skills, spells, and class features. If the creature possesses class features (such as spellcasting or sneak attack) for the class that is being added, these abilities stack. This functions just like adding class levels to a character without racial Hit Dice.
A monster with class levels always possesses treasure equal to an NPC of a level equal to the monster's final CR (as calculated in Step 3, below). To determine the value of this gear, use the value listed for a heroic NPC of that level, as listed in Table NPC Gear. Once a total GP value is determined, follow the rules for outfitting an NPC as outlined in that section. Gear should help a monster with class levels remain challenging and retain statistics close to those presented on Table: Monster Statistics by CR.
Determining the final CR for a creature with class levels requires careful consideration. While adding a class level to a monster that stacks with its existing abilities and role generally adds 1 to its CR for each level taken, adding classes that do not stack is more complicated.
Table: Monsters with Class Levels gives general guidelines regarding which core classes add directly to a monster's abilities based on its role (see Monsters by Role). Classes that are marked “key” generally add 1 to a creature's CR for each level added. Classes marked with a “—” increase a creature's CR by 1 for every 2 class levels added until the number of levels added are equal to (or exceed) the creature's original CR, at which point they are treated as “key” levels (adding 1 to the creature's CR for each level added). Creatures that fall into multiple roles treat a class as key if either of its roles treat the class as key. Note that levels in NPC classes are never considered key.