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In this example, I am trying to calculate the CR of a black pudding [CR 7] that has its HD raised by 5 [Originally 10HD, now 15HD], deals a 5-10 foot AoE splash acid damage whenever damaged via bludgeoning [DC 21 reflex negates], but it raises a good question on whenever I add hit dice to creatures beyond their original starting point, but below their threshold [i.e. 15 HD black pudding] is it standard to add to the base CR of the creature, or are there rules saying that if it isn't above its threshold of HD, it would retain its CR.

If this matters at all, the levels 15-15-14-13 party recently fought the ooze and are confused at the paltry experience.

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Improving Monsters makes it clear that adding HD should come with a corresponding increase in CR, since not only do monsters have more HP and higher attacks and saves, their special abilities may also have higher DCs/caster levels, and they have more skills and feats. For Oozes, this is +1 CR for every four HD, or roughly +1 in this case. The black pudding also gets a higher DC for its acid ability (DC 23: 10+½HD+mod, in this case Con, for 10+7+6), as well as an ability score increase (probably Str, maybe Con).

However, you didn't just add HD, but also a new homebrewed special ability (acid splash). Depending on the damage/effect involved, this seems like it would be worth at least another +1 CR. This would probably need to be tested to be sure, but with more details someone could eyeball it. E.g., a 10' splash that does 2d6 acid damage and DC 23 Reflex or armor dissolve might be as much as +2 CR.

So at the very least, this should be a CR 9, maybe CR 10 monster.

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I confirm TuggyNE's answer. Also I answer to the question in the title: an effective way to calculate the CR of an altered creature has three steps:

  1. use the rules on pg 293 of the Monster Manual

  2. compare with similar monsters

  3. do some tests

You need the steps two and three because these rules, unluckily, aren't so reliable.

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