How to Make a Monster Based on CR (and How to Assign CRs) (Patreon)

A few days ago, I posted asking you all what sort of stuff you'd like to learn about for Fifth Edition. This one comes from Gilyth:

How to make monsters for a specific CR and conversely how to assign a CR to a monster you've already made 

This is a pretty fun one and one that I've mastered over time. 

1 - Know the important charts.

If you ever come into my work office, you'll notice that there are two pieces of paper haning over my desk. On those pieces of paper are five different charts that I use for a lot of my content creation. They are:

• Monster Statistics by Challenge Rating
• Experience Points by Challenge Rating
• Adventuring Day XP
• Encounter Multipliers
• Encounter Difficulty XP Per Character

Of these charts, the most important one for monster crafting is the first one 'Monster Statistics by Challenge Rating.' It is the Rosetta Stone of monster making in Fifth Edition. You can find it on page 274 of the DMG.

2 - Monster CRs come in two main parts: Offensive and Defensive.

The Monster Statistics by Challenge Rating chart has 7 columns.

CR. This works in two ways. It's the CR for the offensive characteristics of the monster as well as the CR for the defensive characteristics of the monster. Monster CRs range from 0 to 30.

Prof Bonus. Like characters, monsters have proficiency bonuses determined by their Challenge Rating. The break points for proficiency bonuses increase at CR 5, CR 9, CR 13, and CR 17. The chart then goes further, increasing at CR 21, CR 25, and CR 29.

Armor Class. Armor class is part of a monster's Defensive CR. Armor Class not only represents the monster's actual AC, but it's "virtual AC" as well. Virtual AC is a bump in the monster's AC from its special abilities. The monster's AC should roughly be in line with its hit points at the CR (see below).

Hit Points. The chief determining factor for a monster's defensive CR is its hit points. The higher the monster's hit points, the more dangerous it is. And the monster's AC should roughly stay in line iwth the monster's hit points at the same CR. For example, a CR 5 monster should have an average of 131 - 145 hit points and an AC of 15.

Attack Bonus. Attack bonus is the monster's total bonus to attack with one of its main attacks (the one it's most likely to use in combat). 

Damage/Round. Like hit points for defensive CR, damage/round is the main determining factor in Offensive challenge rating. Note that the damage is based around the idea that the monster always hits with its attacks and that it attacks with the most damaging way possible. For area of attack features (such as dragon's breath), the attack is considered to deal average damage to 2 creatures. So if a dragon's breath deals 8d6 fire damage, then that's an average of 56 damage dealt by that attack.

Save DC. Finally, if the monster relies more on spells or saving-throw-based attacks than melee or ranged attacks, its save DC is more important than its Attack bonus. Like the Attack bonus, the Save DC should roughly be in line with the damage/round of a monster at that challenge rating. So if a monster deals 23 damage per round on average, its Attack bonus should be around +4 and/or its Save DC should be around 13.

3 - Expected Challenge Rating versus Assigned Challenge Rating.

The DMG gives step-by-step intrusctions for creating monsters with expected challenge ratings. Basically, you assign the CR to start and then reverse engineer your monster to fit that CR. I won't go into detail on how to do that since the rules are there for you to read on page 274 of the DMG.

However, if you want to assign a Challenge Rating to the monster after you've created it, you'll need to factor in all the stuff yourself. Here is the method I do this:

• First, I create the monster. Pretty easy.
• Second, I determine its Defensive CR starting with its hit points. I look at the hit points for the monster and make sure to account for anything that would give it "effective" or "virtual" hit points. For example, Legendary Resistance gives virtual 30 hp per daily usage for creatures of CR 11 or higher. So if my monster has 160 actual hit points but three uses of Legendary Resistance, that makes its "effective" hit points 250, or a defensive CR of 12.
• Once I know its Defensive CR based on its hit points, I cross reference its Armor Class. A CR 12 Monster should have an AC of 17. However, mine has a AC of 19. Because the AC is +2 higher, I adjust the Challenge Rating up one slot (refer to Step 4. on page 274 for details). So now my monster's Defensive CR is 13.
• Next, I slide over to offensive. My monster has a melee attack that deals 65 damage on a hit. It's also got a breath weapon that recharges on a 5-6 and deals 10d6 damage to an area of effect. 
• I calculate the average damage like so: 1 round of the area of effect weapon dealing an average of 70 damage, and 2 rounds of the melee attack that deals 65 damage. Averaging those out, I come up with 66 damage per round. That's an offensive CR of 10 so far.
• Then, like the AC adjustment, I need to make sure my monster's save DC and Attack Bonus is in line with its damage. The monster has a +7 attack bonus and its save DC for its breath weapon is 17. Pretty close, so I won't change the offensive CR anymore. That gives it an offensive CR of 10.
• From there, average the two out. The monster's Defensive CR is 13 and its offensive CR is 10. The average of the two is 11 - 12. I'll stick with 11.
• Double check proficiencies, etc. for the Challenge Rating. Sometimes, assigning the monster a CR can change the entire build of the creature. Make sure to go back through and see if the proficiency bonus made any huge, radical changes. If it did, you might have to recalculate the Challenge Rating.

4 - Make sure you know the things that can affect Challenge Ratings other than damage output and hit points.

Lots of monster powers can affect challenge ratings. Here are a few that are important to remember:

Damage Resistances and Damage Immunities. These have the largest effect on virtual hit points. There is a table on page 277 that shows you what to multiply the effective hit points by. For example, if your monster has immunity to bludgeoning, piercing, and slashing and its expected challenge rating is 11, then you need to multiply its real hit points by 1.5 to come up with its effective hit points when determining its challenge rating.

Special Traits. There is a massive chart on page 280 - 281 of the DMG that shows you how to factor in popular monster features. If you can't find one that matches, try to match it to something close. For example, Avoidance and Evasion are basically the same thing, as are Cunning Action and Nimble Escape.

Flying Monsters. I always forget about this one myself, but if the monster can fly and its CR is 10 or lower, it gets an effective +2 bump to its AC.

Saving Throw Bonuses. Here's another one that I forget a lot: if a monster has three or more saving throw bonuses, its effective AC should be bumped by 2. If it has five or more, increase it by 4.

5 - The basic game design element you should know about.

The game is designed so that a Medium encounter should last three rounds or less. In fact, solo monsters--even if they have significantly higher CRs making them a deadly encounter--rarely last more than 3 rounds. Don't be shocked by this: this is 100% intentional in game design.

Inversely, Player Characters are designed to last 6 - 8 combats before needing a long rest, or roughly 18 - 24 rounds of combat.

. . .

Hopefully, this is helpful. Let me know if you have any questions down in the comments below.