Trouble determining CR or difficulty for homebrew creatures

Question

My dnd campaign is completely homebrew, to the setting, gods, storyline, and even creatures. I did this so I could have more creative freedom, and in creating this world I enjoyed doing so.

My problem is I’m a first time dungeon master and I’m not familiar in assigning a CR to my homebrew creatures or determining if they are too weak or too strong for the party to fight.

If there’s any source of information or criteria I could use to help me in my problem I would greatly appreciate you sharing it.

Disclaimer: Please don’t tell me to use core game creatures because I’m not doing so, it just won’t fit with the campaign and it’s less fun for myself and the players that way.

Answer 1

Regarding your disclaimer:

I don't exactly know what you mean by "won't fit with the campaign", but one of a DM's strongest tools is the reskin. Take the stat block from a monster in one of the books, describe it completely differently, maybe change its creature type or the kind of damage it deals or give it a swim speed, and your players will never know the difference.

You can use a t-rex stat block straight out of the book and claim it's a Tentacled Thrasher, and only you will ever know the truth. I recently took a fire-breathing metal ram (as in a sheep) from the Theros book, and said it was a Sahuagin brawler with a sonic scream attack. It worked just fine and nobody knew I was using a completely un-thematic monster.

You say "it’s less fun for myself and the players", but frankly your players don't know anything. They're just making the numbers go down. They know what you tell them. Interesting and exciting descriptions of what's happening are vastly more important than whether or not the specific block of mechanical stats on the page has been used before.

Yes, it might be fun to make a totally homebrew monster once in a while, but it's frankly shooting yourself in the foot to refuse to use any of the hundreds of pre-made monsters you have available. As a DM, you have limited time to prepare for your game, and building a new monster from scratch is almost never the best use of your time -- let alone an entire adventure's worth of new monsters!

Answer 2

Follow the steps for "Creating a Monster" in Chapter 9 of the Dungeon Master's Guide.

The DMG has an entire section dedicated to this called "Creating a Monster". The introduction reads:

The Monster Manual contains hundreds of ready-to-play monsters, but it doesn’t include every monster that you can imagine. Part of the D&D experience is the simple joy of creating new monsters and customizing existing ones, if for no other reason than to surprise and delight your players with something they’ve never faced before.

The first step in the process is coming up with the concept for your monster. What makes it unique? Where does it live? What role do you want it to serve in your adventure, your campaign, or your world? What does it look like? Does it have any weird abilities? Once you have the answers to these questions, you can start figuring out how to represent your monster in the game.

This section goes on to give detailed steps for estimating a monster's Challenge Rating (the whole section is about 11 pages). In summary, you estimate the monster's Offensive Challenge Rating based on its offensive features, and then estimate its Defensive Challenge Rating based on things like hit points and Armor Class, then average those two for an estimated Final Challenge Rating.

Before getting into the nuts and bolts of Challenge Rating, this section does suggest modifying existing creatures as an alternative to homebrewing wholesale, and I definitely agree with the suggestion. If the theming of published monsters does not fit the theme of the campaign you're running or setting you are working in, my suggestion would be to re-skin or adapt the theme and flavor of some existing monsters to your world, so that you can implement their tried and true mechanics in a way that makes sense for your narrative, and then homebrew a few surprises too.

Answer 3

Frame Challenge: D&D might not be the right game here

Look, if you're going to rework everything, the setting and monsters included, and don't want to use ANY of the existing content maybe Dungeons and Dragons isn't the right tool for your game. There are plenty of other options out there, from Generic settings to specific ones. GURPS, Savage Worlds, Swords without Master all might fit the bill, or maybe a Powered By the Apocalypse or Forged in the Dark hack would work better. Part of the reason to pick a particular system to run your game is to make it easier for everyone to play the game.

The CR system in D&D is already pretty terribly balanced, so if you're homebrewing everything you're only making your life needlessly difficult and increasing the number of times you'll have to fudge to avoid TPKs AND the number of times fights will be boring cake walks.

Basically it comes down to if you want to play D&D you should play D&D and if you don't want to play D&D you should play something else. If you don't want to play/run D&D but do it anyways you're just going to have a bad time, or at least, a suboptimal one.

Answer 4

You can reduce the DMG monster CR guidelines, for CR 1+ creatures, to a pretty simple formula.

Add up at-will damage per round, divide by 12 (ignore miss chance). Add up HP, divide by 32. Add up "one-shot" damage above at-will damage, divide by 36. Add the primary stat bonus for attacks divided by 4. Add AC divided by 4. Then subtract 4.

That gives you the CR for a creature with no status effects or special abilities.

The DMG provides rules for handling legendary resists, advantage, invisibility, magic resistances, area effects, etc.

While this system doesn't seem that similar to the DMG rules, I find it quicker and simpler to tweak monsters.

Suppose you have a creature that that has 252 HP, has 22 intelligence, makes 3 Int-based attacks for 3d12+6 psychic damage against AC, and 1/encounter can deal 2d12 (save for half) thunder damage when damaged to everyone within 200' (when "bloodied"); save DC is int-based. It has 17 AC (natural armor). It has magic resistance and 3 strong saves.

What CR do we get?

+6/4 (Int) + 17/4 (AC) + 76.5/12 (at-will damage) + 39/36 (AOE extra damage). MR is worth +2 AC (+0.5 CR), 3 good saves is worth +2 AC (+0.5 CR).

Total is 10.2, so CR 10 monster. From its CR, it has a +4 proficiency bonus. So its (Int-based) attack bonus is +10, and its save DC is 18.

(For fancy stuff from the DMG, if it boosts "defensive CR", add 1/2 that amount to total CR. If it boosts effective AC, add 1/4 that amount to total CR.)

For monsters above CR 20, you have to divide the CR by 2 and add 10.

...

This doesn't help with sub-1 CR monsters at all. But it does make higher level monsters really quick to measure.

For sub-1 CR monsters, I'd advise reskinning. Because they should mostly be a pool of AC, HP, a single attack with moderate damage, and maybe 1 special trick.

Taking an existing low-CR monster and reskinning, possibly tweaking abilities, is probably best for those cases. If you are doing anything seriously weird with sub-1 CR monsters you'll have to do real playtesting anyhow.

Answer 5

There are various methods for this, but they are fairly inaccurate.

I have found by far the best way to estimate the chances of a new stat block or ability to be to simulate it very briefly with some back-of-the-envelope math. Expected damage vs AC/save of the party in question is a very strong way to do this in 5e, as hp damage is a major part of the game.

You can likewise simulate save-or-be-disabled abilities (the second most important thing on any statblock usually) by number of turns until the character breaks free (or if they can't break free, treating it as a one-hit knockout based on their chances to save - a full minute of paralysis or whatever usually means you're out of that fight) x the expected damage of the creature/turn. So if the creature hits for 5 damage/turn typically, and the average party member will be paralyzed for 2 rounds if they fail their initial save, it's around 10 damage for a failed save (and the creature's actions).

The big advantage of this method is that you can do very cursory math (damage/round if everything hits) and it's helpful, or you can do actual expected damage (attack vs AC x damage/round), and it's helpful, or you can think about how the fight might go in terms of movement, positioning, saving throws etc. and compare it to the specifics of your party's actual character sheets, and it's helpful.

These are all very similar actions - doing any of them gets you better at doing all of them - and doing the quickest one through to the most laborious one is all helpful. It's a sliding scale of effort you can adapt to your prep time and situation, and you get faster at it over time (much faster) as it's mostly done in your head.

A few other advantages -

• You internalize some of the creature's numbers, often making combat more fluid as you need to reference things less.
• You are comparing it to your party's actual stats, which often do not resemble the stats many other people expect (parties are often wildly different), thus making your comparisons/tests more accurate.
• You are more likely to find the major cause of TPK - monster strength aimed at party weakness - than any generic CR calculator (eg - a party of low Con save guys vs a creature that paralyzes on save ends in a large area-of-effect vs Con. Most parties have high Con, or at least a few high Con guys, but some don't. Likewise low Wis parties, low hp parties, low damage parties, etc. The most common cause of TPK is a monster with one powerful ability that is aimed squarely at a collective weakness)
• Simulating the fight can give you ideas in terms of terrain or choreography to either favour the monster (making it a more interesting and difficult fight) or set up situations for the party to be awesome, such as by realizing that the monster's burrowing abilities would make it a terror to fight underground and that dovetails well with your plans for the dwarf king's keep when combined with the party's lack of enemy sensing abilities etc.

Answer 6

Ok, here's the simple answer paraphrased from the DMG. You'll need to read the whole details, but this is the basic process.

1. Write down the AC, eHP (effective HP), Attack Bonus, and DPR (Damage per Round) of your monster
2. Consult the table on page 274 of the DMG find the row corresponding to the HP of your monster. Write down the CR of that row.
3. Now look at the AC listed next to that eHP. If the AC of your monster is at least 2 higher or lower than the listed AC, adjust the CR up/down by 1 for every 2 points of difference. Write down that adjusted CR. This is called the Defensive CR of a monster.
4. Now do the same with damage per round, find the row, write it down, look at the attack bonus, if it's 2 higher or lower adjust the CR up/down by 1 for every 2 points of difference. Write down that adjusted CR, this is the Offensive CR of a monster.
5. Average the Defensive and Offensive CR to get the CR of the monster.

CR	eHP	AC	DPR	Attack Bonus
0	1	13	0	3
1/8	7	13	2	3
1/4	36	13	4	3
1/2	50	13	6	3
1	71	13	9	3
....

As an example let's look at Goblins;

1. eHP 7, AC 15, DPR 5, Hit +4
2. 7 eHP is CR 1/8.
3. The listed AC is 13, our AC is 15 so that's 2 higher, adjust the CR by 1 row to 1/4. That's our Defensive CR.
4. The same for DPR; we have 5 DPR so that's 1/4, the listed hit bonus is +3 and ours is +4 so that's close enough. Our Offensive CR is 1/4.
5. Both are 1/4 so the average is CR 1/4

Answer 7

The DMG has guidelines for this, but they are wrong.

That is to say, they don't match the creatures in the monster manual. For example, the table on page 274 of the DMG shows CR 1 creatures averaging 78 HP, but if you look at the hit points of all the CR 1 SRD entries, the average is 28. CR has been troubled since it's inception in previous editions, especially with the lack of quantification or even consideration for special abilities, especially intangible ones that produce effects, not simply more damage, but it can still be helpful, especially if you use guidelines that match the monster manual.

The blog of holding did a thorough 5e analysis, but Darth Pseodonym is right, your easiest route is to use existing stat blocks for your custom monsters. If you do end up writing completely custom creatures, I suggest using the blog of holding's monster manual on a business card. Start simple, by just setting the starting CR using one trait, like hit points, and then make sure your other statistics line up, or adjust the CR accordingly.

One can only speculate why the DMG guidelines in chapter 9 aren't consistent with the entries in the monster manual.