People complain that the DMG CR stuff doesn't work. And it really doesn't work for the use most people put it to.
So here is a system based off the DMG CR stuff, but upside down. Instead of starting with stats and generating CR, this starts with CR and gives you a budget to buy stats. The result should be within a few percent of doing it the other way.
Warning: I made no attempt to make this work for sub-1 CR monsters. There is probably a budget we can use for them, I just didn't work it out (yet).
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Simple Monster Building
Start with a CR. Your CR determines your proficiency bonus and your MBP (monster building point) budget.
You get 14 MBP "base" plus 4 MBP for every CR up to 20, and 12 for every CR beyond 19. (CR 20+ monsters get 3xs of HP and damage per additional CR in DMG chart)
You also get 1 bonus MBP at CR 5, 11, and 17 (because I needed a few more MBP at 20 to make the math work, and doing it at the tier barriers makes sense).
Now work out the monster's stats.
For Free you get:
0 standard attack
10 AC
10(+0) in your attack stat (spellcasting stat, or weapon)
0 HP
A base template, to start with something halfway usable, might be:
12 standard attack damage (4)
13 AC (3)
16 in your attack stat (3)
32 HP (4)
This costs you your 14 "free" MBP.
To calculate your attack bonus/save DCs, use your attack stat plus your proficiency bonus.
Note that your attack stat, if it adds to damage, must be paid for in the damage calculation.
Now you spend your MBPs:
Things to buy:
8 HP for each 1 MBP
+/- 1 AC for +/- 1 MBP (can even sell AC! But don't go crazy.)
+3 standard attack routine damage for 1 MBP
+3 legendary action/off turn/bonus attack damage for 1 MBP (total damage; 3 legendary actions doing 10 each is 30/3 = 10 MBP)
+2 primary attack/save DC attribute (a +1 attribute modifier) for 1 MBP (or -2 to get 1 back). (if you have more than one, average them. If one is used way more than the other, ignore the other one).
9 points of single-use damage for 1 MBP. If this replaces your standard attack attack routine, only pay for the damage difference.
A save proficiency is 1 MBP.
Legendary resists cost 2 MBP for the first, then 1 for each one afterwards (up to 3).
Significant resistances/immunities doubles the cost of buying HP (or more or less)
Significant vulnerabilities halves HP cost
Flying at or under 9 CR is worth 2 MBP
Attacks that are area effects (1/2 damage on save) should be assumed to hit more than 1 target; multiply their effective damage before calculating their cost.
Various other abilities have costs. You can wing it, or you can look them up in the monster building rules in the DMG and translate it to AC/ATK (at 1 MBP per AC/ATK).
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I'm going to make a CR 5 moon griffon. I get 35 points (14 + 5*4, plus 1 bonus for CR 5).
I start with a Griffon
It gets 18 strength (4 MBP)
It flies (2 MBP)
It has 12 AC (2 MBP)
Its attacks deal 1d8+4 radiant damage (beak) and 2d6+4 radiant damage (claws) (19.5) (6.5 MBP)
It has 59 HP (7.4 MBP)
(note: 21.9 MBP; a CR 2 monster gets 22, so the Griffon is CR 2, as expected)
Then I boost it:
It is invisible unless in sunlight (4 MBP for advantage on both attacks and defenses; I assume it doesn't engage in sunlight. PCs with sunlight powers are rare at level 5ish)
It is resistant to weapon damage and immune to radiant damage (7.4 MBP from double HP cost)
Radiant Aura: creatures who on their turn approach within 5' or end their turn there, take 1d10 radiant damage (con save 16 DC for half) 3.7 MBP, discounted by 2 MBP because its effective use is exclusive with flying (1.7 MBP))
Total spent: 35 MBP
There we go, a moon griffon.
Math:
Some simple templates. You can start with a CR X bruiser, and add stuff to it out of the above budgets.
CR 2 bruiser. 22 MBP budget
AC 15 (5 MBP)
52 HP (6.5 MBP)
+4 attack stat (4 MBP)
2 attacks, each for 1d10+4 (19 DPR) (6.3 MBP)
Total. 21.8 MBP
CR 5 bruiser. 35 MBP budget.
AC 18 (8 MBP)
+5 attack stat (5 MBP)
100 HP (12.5 MBP)
2 attacks, each for 2d8+5 (28 DPR) (9.3 MBP)
Total: 34.8 MBP
CR 11 bruiser. 60 MBP budget.
AC 20 (10 MBP)
+6 attack stat (6 MBP)
150 HP (18.8 MBP)
2 good saves (2 MBP)
3 attacks, each for 3d6+6 (49.5 DPR) (16.5 MBP)
special attack (10d6, large aoe, DC 18) (105 Damage-49.5) (6.2 MBP)
Total: 59.5 MBP
CR 17 bruiser. 85 MBP budget.
AC 21 (11 MBP)
+8 attack stat (8 MBP)
230 HP (28.8 MBP)
4 good saves (4 MBP)
3 attacks, each for 2d12+8 (63 DPR) (21 MBP)
special attack (10d10, large aoe, 21 DC)(165 damage-63) (11.3 MBP)
Total: 84.1 MBP spent
CR 20 bruiser. 105 MBP budget
AC 23 (13 MBP)
+9 attack stat (9 MBP)
250 HP (31.3 MBP)
4 good saves (4 MBP)
3 attacks, each for 3d12+9 (85.5 DPR) (28.5 MBP)
special attack (bonus action, large aoe, 23 DC) 10d10 (165 dam) (18.3 MBP)
Total: 104.1 MBP spent
CR 25 bruiser. 165 MBP budget
AC 25 (15 MBP)
+10 attack stat (10 MBP)
400 HP (50 MBP)
5 good saves (5 MBP)
3 legendary resists (4 MBP)
Magic resistance (2 MBP)
3 attacks each for 3d12+10 (88.5 DPR) (29.5 MBP)
3 legendary actions each for 3d12+10 (88.5 DPR) (29.5 MBP)
special attack (bonus action, large aoe, 23 DC) 10d12 (195 dam) (21.7 MBP)
Total: 166.7 MBP spent
So here is a system based off the DMG CR stuff, but upside down. Instead of starting with stats and generating CR, this starts with CR and gives you a budget to buy stats. The result should be within a few percent of doing it the other way.
Warning: I made no attempt to make this work for sub-1 CR monsters. There is probably a budget we can use for them, I just didn't work it out (yet).
---
Simple Monster Building
Start with a CR. Your CR determines your proficiency bonus and your MBP (monster building point) budget.
You get 14 MBP "base" plus 4 MBP for every CR up to 20, and 12 for every CR beyond 19. (CR 20+ monsters get 3xs of HP and damage per additional CR in DMG chart)
You also get 1 bonus MBP at CR 5, 11, and 17 (because I needed a few more MBP at 20 to make the math work, and doing it at the tier barriers makes sense).
Now work out the monster's stats.
For Free you get:
0 standard attack
10 AC
10(+0) in your attack stat (spellcasting stat, or weapon)
0 HP
A base template, to start with something halfway usable, might be:
12 standard attack damage (4)
13 AC (3)
16 in your attack stat (3)
32 HP (4)
This costs you your 14 "free" MBP.
To calculate your attack bonus/save DCs, use your attack stat plus your proficiency bonus.
Note that your attack stat, if it adds to damage, must be paid for in the damage calculation.
Now you spend your MBPs:
Things to buy:
8 HP for each 1 MBP
+/- 1 AC for +/- 1 MBP (can even sell AC! But don't go crazy.)
+3 standard attack routine damage for 1 MBP
+3 legendary action/off turn/bonus attack damage for 1 MBP (total damage; 3 legendary actions doing 10 each is 30/3 = 10 MBP)
+2 primary attack/save DC attribute (a +1 attribute modifier) for 1 MBP (or -2 to get 1 back). (if you have more than one, average them. If one is used way more than the other, ignore the other one).
9 points of single-use damage for 1 MBP. If this replaces your standard attack attack routine, only pay for the damage difference.
A save proficiency is 1 MBP.
Legendary resists cost 2 MBP for the first, then 1 for each one afterwards (up to 3).
Significant resistances/immunities doubles the cost of buying HP (or more or less)
Significant vulnerabilities halves HP cost
Flying at or under 9 CR is worth 2 MBP
Attacks that are area effects (1/2 damage on save) should be assumed to hit more than 1 target; multiply their effective damage before calculating their cost.
Various other abilities have costs. You can wing it, or you can look them up in the monster building rules in the DMG and translate it to AC/ATK (at 1 MBP per AC/ATK).
---
I'm going to make a CR 5 moon griffon. I get 35 points (14 + 5*4, plus 1 bonus for CR 5).
I start with a Griffon
It gets 18 strength (4 MBP)
It flies (2 MBP)
It has 12 AC (2 MBP)
Its attacks deal 1d8+4 radiant damage (beak) and 2d6+4 radiant damage (claws) (19.5) (6.5 MBP)
It has 59 HP (7.4 MBP)
(note: 21.9 MBP; a CR 2 monster gets 22, so the Griffon is CR 2, as expected)
Then I boost it:
It is invisible unless in sunlight (4 MBP for advantage on both attacks and defenses; I assume it doesn't engage in sunlight. PCs with sunlight powers are rare at level 5ish)
It is resistant to weapon damage and immune to radiant damage (7.4 MBP from double HP cost)
Radiant Aura: creatures who on their turn approach within 5' or end their turn there, take 1d10 radiant damage (con save 16 DC for half) 3.7 MBP, discounted by 2 MBP because its effective use is exclusive with flying (1.7 MBP))
Total spent: 35 MBP
There we go, a moon griffon.
Math:
So we start by noting that each +15 HP and +6 damage ups your offensive/defensive CR by 1.
Each +2 AC and +2 ATK ups your offensive or defensive CR by 2. And CR is the average of offensive and defensive; so +4 AC or +4 ATK is worth 1 CR.
I'll invert this, and make each CR be worth 4 monster building points, which you then can buy ATK and AC with.
We now just need to get CR 19 and CR 1 to line up; the middle does a pretty good job on its own.
CR 1 is 78 HP, 13 AC, 11.5 damage, and +1 ATK over proficiency.
CR 19 is 348 HP, 19 AC, 119.5 damage, and +4 ATK over proficiency.
That is +270 HP, +6 AC, 108 damage and +3 ATK for 79 MBP.
We subtract out the AC and ATK (already set at 1 each) giving us 70 MBP to buy 270 HP and 108 damage.
Now, the chart has 6 damage for 1 offensive CR. 3 is half that. Lets do 108/3 is 36 MBP worth of damage, leaving 34 MBP to buy 270 HP.
270/34 is 7.9. Call it 8 HP per MBP.
Next we start with the CR 1 monster.
78/8 is 9.75 MBP for HP
11.5 damage is is 3.8 MBP for damage
10+3 AC is 3 MBP
+3 ATK is 1 above proficiency, for 1 MBP.
Total is 17.55. We need to shave this down so you can afford CR 1 with 4 MBP.
We round this to 18 MBP, giving us 14 MBP + 4 per CR.
The sample is just aimed giving a reasonable starting set of stats.
16 strength, 12 damage, 32 HP, 13 AC.
Note that when doing this review of the math, I noticed an error; the original price of HP was 7 per MBP, and I adjusted it to 8 after I spotted my error.
Each +2 AC and +2 ATK ups your offensive or defensive CR by 2. And CR is the average of offensive and defensive; so +4 AC or +4 ATK is worth 1 CR.
I'll invert this, and make each CR be worth 4 monster building points, which you then can buy ATK and AC with.
We now just need to get CR 19 and CR 1 to line up; the middle does a pretty good job on its own.
CR 1 is 78 HP, 13 AC, 11.5 damage, and +1 ATK over proficiency.
CR 19 is 348 HP, 19 AC, 119.5 damage, and +4 ATK over proficiency.
That is +270 HP, +6 AC, 108 damage and +3 ATK for 79 MBP.
We subtract out the AC and ATK (already set at 1 each) giving us 70 MBP to buy 270 HP and 108 damage.
Now, the chart has 6 damage for 1 offensive CR. 3 is half that. Lets do 108/3 is 36 MBP worth of damage, leaving 34 MBP to buy 270 HP.
270/34 is 7.9. Call it 8 HP per MBP.
Next we start with the CR 1 monster.
78/8 is 9.75 MBP for HP
11.5 damage is is 3.8 MBP for damage
10+3 AC is 3 MBP
+3 ATK is 1 above proficiency, for 1 MBP.
Total is 17.55. We need to shave this down so you can afford CR 1 with 4 MBP.
We round this to 18 MBP, giving us 14 MBP + 4 per CR.
The sample is just aimed giving a reasonable starting set of stats.
16 strength, 12 damage, 32 HP, 13 AC.
Note that when doing this review of the math, I noticed an error; the original price of HP was 7 per MBP, and I adjusted it to 8 after I spotted my error.
Some simple templates. You can start with a CR X bruiser, and add stuff to it out of the above budgets.
CR 2 bruiser. 22 MBP budget
AC 15 (5 MBP)
52 HP (6.5 MBP)
+4 attack stat (4 MBP)
2 attacks, each for 1d10+4 (19 DPR) (6.3 MBP)
Total. 21.8 MBP
CR 5 bruiser. 35 MBP budget.
AC 18 (8 MBP)
+5 attack stat (5 MBP)
100 HP (12.5 MBP)
2 attacks, each for 2d8+5 (28 DPR) (9.3 MBP)
Total: 34.8 MBP
CR 11 bruiser. 60 MBP budget.
AC 20 (10 MBP)
+6 attack stat (6 MBP)
150 HP (18.8 MBP)
2 good saves (2 MBP)
3 attacks, each for 3d6+6 (49.5 DPR) (16.5 MBP)
special attack (10d6, large aoe, DC 18) (105 Damage-49.5) (6.2 MBP)
Total: 59.5 MBP
CR 17 bruiser. 85 MBP budget.
AC 21 (11 MBP)
+8 attack stat (8 MBP)
230 HP (28.8 MBP)
4 good saves (4 MBP)
3 attacks, each for 2d12+8 (63 DPR) (21 MBP)
special attack (10d10, large aoe, 21 DC)(165 damage-63) (11.3 MBP)
Total: 84.1 MBP spent
CR 20 bruiser. 105 MBP budget
AC 23 (13 MBP)
+9 attack stat (9 MBP)
250 HP (31.3 MBP)
4 good saves (4 MBP)
3 attacks, each for 3d12+9 (85.5 DPR) (28.5 MBP)
special attack (bonus action, large aoe, 23 DC) 10d10 (165 dam) (18.3 MBP)
Total: 104.1 MBP spent
CR 25 bruiser. 165 MBP budget
AC 25 (15 MBP)
+10 attack stat (10 MBP)
400 HP (50 MBP)
5 good saves (5 MBP)
3 legendary resists (4 MBP)
Magic resistance (2 MBP)
3 attacks each for 3d12+10 (88.5 DPR) (29.5 MBP)
3 legendary actions each for 3d12+10 (88.5 DPR) (29.5 MBP)
special attack (bonus action, large aoe, 23 DC) 10d12 (195 dam) (21.7 MBP)
Total: 166.7 MBP spent
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