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The guidelines for making your own monsters are based around the Monster Statistics by Challenge Rating table on p274. Inspired by this question regarding Animated Armor (MM 19), which is CR1, let's see where the Armor would be if we made it from scratch...

Defensive CR: The Armor has 33 HP, but step 9 (Damage Vulnerabilities, Resistances and Immunities, DMG 277) has us adjust effective HP based on resistance.

So we start with 33 HP, giving us CR 1/8, which tells us that we're using the first line of Effective Hit Points Based on Resistances and Immunities, for an x2 multiplier for each Resistance or Immunity.

The Armor has two Immunities, so has 33x2x2 or 132 "effective" hit points. That puts us at CR 5. We then look at its AC of 18, which is 3 higher than the normal 15 for CR 5. That adjusts us up by one, to CR 6.

Offensive CR: It does 10 (2x5) points of damage, for a CR of 1. Its Attack Bonus is +4, which is 1 higher than the normal +3, but doesn't adjust CR up.

Final CR is the average of the two, or (6+1)/2. That gives us 3.5, which we round against the players, for a final final CR 3.

How is Animated Armor a CR1 challenge? Its only notable disadvantage (from a combat perspective) is its Antimagic Susceptibility, but that has no effect on CR, per DMG 280. What's knocking it down from CR 3 to CR 1? Or am I horribly misunderstanding how the creation rules work?

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Are the monsters in the 5e MM actually following the monster creation rules in the 5e DMG?

Short answer: No.

Longer answer:

Paul Hughes, of blogofholding, has written a series of blog posts investigating this particular question. His approach was one of doing a statistical analysis of all the monsters in the MM (and some other books). He arrived at several significant findings:

Paul also references an interview by Mike Shea (aka Sly Flourish) of D&D lead rules designer Jeremy Crawford about monster design:

Mike asked Jeremy about the table in the DMG for homebrewing monsters. Jeremy provided this interesting fact: apparently, the canonical formula for determining monster CR is encoded in an internally-used Excel spreadsheet. (We’ve seen this spreadsheet in action in the Mike Mearls Happy Fun Hour.) The table in the DMG was made after the spreadsheet, and was an attempt to reverse-engineer and simplify the spreadsheet’s formulas for DM’s home use: it’s not used as part of the process for creating for-publication monsters.

So, what are the more accurate rules?

Paul has summarised his findings in a reverse-engineered set of 5e monster creation rules that more closely mirrors the results apparent in the MM et al. He has both a one-page set, and a two-sides-of-a-business-card summary.

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    \$\begingroup\$ OMG including these references in this answer was completely awesome. \$\endgroup\$ – Jack Aug 15 '18 at 20:36
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First thing to remember is that the monster creation rules are meant to be used as a general guideline. The monsters that already exist in the printed text have had the benefit of playtesting which may have adjusted their CR up or down.

In regards to the animated armor, I think there is a mistake in your math, specifically with the hp based on CR (33hp is 1/8 not 1/4) and the immunities. In the paragraph above the Effective Hit Points Based on Resistances and Immunities table it says this:

Giving a monster resistances and immunities to three or more damage types (especially bludgeoning, piercing, and slashing damage) is like giving it extra hit points.

So two immunities may not count at all, especially when they're less common damage types like poison and psychic. Also, it does not say multiple immunities are multiplicative. In it's own example of resistance to all non-magical weapon types, it only multiplies once.

With this in mind, the challenge rating would be between .75 (rounded to one) if you don't count the immunities or 1.5 if you do.

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  • \$\begingroup\$ Thanks for catching my error on hit points - I'll edit to reflect that. Regarding the damage immunities not stacking, that makes sense, although it's not entirely clear in the text. The part about not multiplying HP unless there are three or more immunities is really easy to miss. \$\endgroup\$ – kevin.matheny Mar 28 '15 at 18:17

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