[RPG] How to convert the ECL of NPC’s to CR

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My campaigns are RP heavy, and the players fight NPC's as often as monsters. Hence, how could I calculate the CR of something with class levels?

Best Answer

The Challenge Rating of a creature with class levels equals the creature's listed Challenge Rating plus the creature's levels in associated classes and half (mostly) the creature's levels in nonassociated classes

For many creatures, the DM picks whether a creature's class levels are associated or nonassociated, but sometimes the creature's description will indicate what classes are associated and which are nonassociated.

  • The Monster Manual on Associated Class Levels says

    Class levels that increase a monster’s existing strengths are known as associated class levels. Each associated class level a monster has increases its CR by 1.

    Barbarian, fighter, paladin, and ranger are associated classes for a creature that relies on its fighting ability. For example, if you add a level of fighter, barbarian, ranger, or paladin to a frost giant, this directly improves the monster’s existing strengths and is therefore an associated class level.

    Rogue and ranger are associated classes for a creature that relies on stealth to surprise its foes, or on skill use to give itself an advantage. The babau demon, for example, is “sneaky and sly” and has sneak attack as a special ability. Rogue is an associated class for this creature.

    A spellcasting class is an associated class for a creature that already has the ability to cast spells as a character of the class in question, since the monster’s levels in the spellcasting class stack with its innate spellcasting ability. A rakshasa, for example, casts spells as a 7th-level sorcerer. If it picks up a level of sorcerer, it casts spells as an 8th-level sorcerer. (294)

  • And on Nonassociated Class Levels says

    If you add a class level that doesn’t directly play to a creature’s strength (such as adding a sorcerer level to a frost giant), the class level is considered nonassociated, and things get a little more complicated. Adding a nonassociated class level to a monster increases its CR by 1/2 per level until one of its nonassociated class levels equals its original Hit Dice. At that point, each additional level of the same class or a similar one is considered associated and increases the monster’s CR by 1.

    For example, frost giants have 14 HD. After you add 14 levels of sorcerer to a frost giant (and +7 to its CR), any further sorcerer class levels are considered associated. Adding one more sorcerer level increases this particular frost giant’s CR by 1.

    Levels in NPC classes are always treated as nonassociated. (ibid.)

For example, let's take the bugbear:

  • A bugbear sorcerer 3 is CR 3 (2 from bugbear + (3 from sorcerer / 2 nonassociated = 1.5 rounded down)).
  • A bugbear bugbear sorcerer 10 is CR 10 (2 from bugbear + (3 from sorcerer / 2 nonassociated = 1.5 rounded down) + 7 from sorcerer).
  • A bugbear ranger 3 is CR 5 (2 from bugbear + 3 from ranger).
  • A bugbear ranger 10 is CR 12 (2 from bugbear + 10 from ranger).
  • A bugbear ranger 3 / sorcerer 3 is CR 6 (2 from bugbear + 3 + (3 from sorcerer / 2 nonassociated = 1.5 rounded down)).
  • A bugbear ranger 10 / Sorcerer 10 is CR 20 (2 from bugbear + 10 + (3 from sorcerer / 2 nonassociated = 1.5 rounded down) +7 from sorcerer).
  • A bugbear sorcerer 20 is CR 20 (2 from bugbear + (3 from sorcerer / 2 nonassociated = 1.5 rounded down) + 17 from sorcerer).

Caution!

This can get weird at high levels with a liberal DM, who can determine that, for example, a storm giant's 15 psion levels are nonassociated making the storm giant psion 15 CR 20, which, to some, might seem a little low. But, really, when using these rules, at the levels a storm giant psion 15 or a like monster could be appropriate, it's for the DM's to determine whether a creature is actually an appropriate challenge for the PCs, the system, at that point, only providing guidelines.

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