Tuesday, 29 July 2014

D&D 5e Monsters: Part 3: Construction: CR & Size

D&D 5e Monsters: Part 3: Construction: CR & Size

Surf gets side-tracked for a while... But that's a good thing!


Part 3: Construction: CR & Size

OK I want to open this article with a couple of apologies.

First up, some folks felt that "two introductions" for this series was excessive. I can understand this and I'd normally agree - be assured that I agonised over this when I was writing Before Building. But in the end I decided that a process-oriented build really does demand that the conceptual pre-work be done. It is far, far more important for this kind of build than for a result-oriented build. To those who felt put-out I do apologise and can assure you that it is not how I prefer to write a series of articles, so you shouldn't expect a repeat any time soon.

Secondly, I apologise for the lateness of this article. I originally indicated I'd post it almost a week ago, but as I started writing Part 3 I realised how fundamental CR is to the process. I was faced with a choice - do some quick and dirty analysis to work around it or do some solid analysis. So I chose the latter and disappeared down a rabbit hole for a while.

I think most readers will be happy that I did... I certainly hope so!

Let's have a look at the two foundational components of a D&D 5e monster - CR and Size. Then we'll use these to fill out some peices of an example monster.


Challenge Rating

So I had committed to leaving Challenge Rating (aka "CR") analysis until the Monster Manual hit the shelves, but as I dug into 5e monsters I developed a deeper appreciation of something I had previously suspected. While CR clearly is a measure of how appropriate a given creature is for a same-level party to fight, it is also a foundational element in building 5e monsters. When we decide to build an opponent for the PCs, regardless of the approach we use, we have an idea of how challenging it should be. We may or may not have formalised it, but we do have an idea. I starting thinking of this as Target CR in order to differentiate it from the actual final CR.

I came to this realisation as I was doing some analysis to work around using CR when building monsters. Ultimately this lead me to analyze PC data earlier than anticipated. Interested readers might check out this thread on the wizards.com forums where I posted the data I crunched about Classes.

Several key components of monsters are based directly on Target CR...

A monster's Hitpoints are the result of it's number of Hit Dice, it's size and it's Ability Score modifiers. But how many hitpoints should my monster have? Target CR yields a Target HP range towards which we can build.

 ✝ Not in the Starter Set

Monster Damage output results from a creature's traits and actions. A combat-focussed creature's damage is influenced by it's Ability Score modifiers, it's size, the weapon it uses, the number of attacks it makes and several other factors. A creature that focusses on casting spells mainly produces damage according to it's caster level. Creatures with other approaches produce their damage output via other methods. But what should this damage output be? Target CR gives us an appropriate range for this.

Proficiency for monsters is indexed directly off the creature's CR. We can simply use the Character Advancement table on page 10 of the Basic D&D v0.1 and use CR instead of level.

Ability Scores also appear to scale based on CR. We'll look at this more closely in Part 4: Construction: Abilities & HP.

Experience Points seem to be tightly bound to CR. While there are a number of gaps the XP rewards for the monsters presented in the Starter Set make it quite easy to fill in these gaps up to level 10 with a reasonable level of confidence. We can estimate values for the missing CRs using the following moderate polynomial:
  XP = -0.3553xCR^3 + 60.485xCR^2 + 5.5534xCR + 155.52

It's important to note that our limited sample size makes it difficult to be confident of certain numbers past a reasonable level or CR. In this case we can have a high level of confidence that we are in the right ballpark with these XP numbers. Some of them might be a hundred XP or so out but they'll be pretty close and more than good enough for most uses. We'll revisit these as soon as there's more data, but it's what we have to use for now.

Note that I won't provide tables out past CR10 until we have at least some examples of creatures at these higher levels to analyze.

Prediction: I believe we'll see creatures up to at least CR25 in the Monster Manual, probably out to CR30 and maybe even beyond.



Monster Size partners with Target CR to provide targets for most of the building blocks of a 5e monster, either directly or indirectly.

Size has a direct bearing on Hit Points (via Hit Dice), weapon damage dice and on speed. It may also influence other aspects of monsters.

 ✝ Not in the Starter Set

Page 71 of the Basic D&D v0.1 PDF outlines the appropriate size categories in D&D 5e and Appendix B of the Starter Set shows us the HD size for most of these. Appendix B doesn't tell us the HD size for Huge or Gargantuan creatures, but it's pretty reasonable to assume that these are simply the next dice sizes up from Large.

Speed is normally thought of as an aspect of race - it's listed in the Racial Traits section of the Basic D&D v0.1 PDF, for example. I believe that this is primarily because Size is a component of Race. Note that Speed is not a "hard" aspect of size, rather it is a starting point that can be modified and tuned to suit the specific creature you are building. Variance up to one size category above or below seems to be quite normal.

Prediction: Because of the relationship between Size and Hit Points I believe monsters of the smaller sizes will become less frequent as CR increases and that creatures past say CR10 will be predominantly Medium or bigger. We'll need the volume of data expected in the Monster Manual to confirm this.


Example: Human Pyromancer

Since this part of my blog focuses on the build-oriented approach to D&D 5e monster creation I thought it would be appropriate to provide some example monsters for illustration purposes. In these examples we'll normally omit parts of the stat block which we have not yet dealt with.

Human Pyromancer

Medium humanoid (human), any alignment

Speed: 30ft

Challenge 5 (1,700 XP)

The idea behind the Human Pyromancer is a "Glass Cannon" ranged attacker with an aracane fire theme. The intent is for this monster to blast the PCs from a distance, while receiving protection from melee type monsters and cover. Once the PCs get up close it should be all over for this guy. With some lower level meat-shield support it could be the central piece of a toughish level 5 or 6 encounter. It should have some longevity out to level 10 or so as lower-end ranged support for higher-CR melee creatures.

From a monster design perspective it will allow us to explore how spellcasting monsters are built in D&D 5e and thus also explore the math associated with casters. It will also allow us to illustrate the flexibility the CR guidelines for HP and Damage support.