How do I determine a dragon's new Challenge Rating when applying the Shadow Dragon template?

Question

The Monster Manual contains a template whereby any true dragon may be turned into a shadow dragon. A shadow dragon differs from a true dragon in the following ways:

• It has resistance to necrotic damage always, and resistance to all other forms of damage except force, psychic and radiant, when in dim light or darkness.
• It has double proficiency in Stealth.
• It can Hide as a bonus action when in dim light or darkness.
• It has sunlight sensitivity (disadvantage on attack rolls and Perception checks involving sight when in sunlight).
• The damage type of both the non-piercing portion of its bite, and its breath weapon, is necrotic (regardless of type of dragon).
• Any humanoid brought to 0 hit points by its breath weapon is automatically killed (no death saving throws), and a shadow rises from its corpse.

Otherwise, the shadow dragon has the same stats as the true dragon. How do you evaluate these differences to come up with a new Challenge Rating? Right now I am specifically looking for the CR of a Black Shadow Dragon Wyrmling, but the best answer will explain how to determine the CR of all types of shadow dragon.

One clue: The Monster Manual provides one example stat block for a shadow dragon: a Young Red. The CR of a Young Red Shadow Dragon is 13, while the CR of a Young Red Dragon is 10. Is merely adding 3 to the CR a good rule of thumb?

Answer 1

I know I'm a little late to the party here (18 months since OP) but I was searching for this answer also.

Carcer pointed to directions in the DMG, and following them I built a formula that calculates a creature's CR (More or less. It's not an exact science) from their AC, HP, Attack Bonus, Damage per Round, and whether they have significant resistances. I punched in the stats for all the dragons and was pretty happy with the results. I've attached an image of the output table here, in case anyone else stumbles onto this thread like I did.

Thanks Purple Monkey for showing me how to make a MathJax table:

Header Key: W = Wyrmling. Y = Young. A = Adult. N = Ancient. S = Shadow Dragon. L = Dracolich.

\begin{array}{r|llllllllll} \text{CR by Type} & \text{W} & \text{WS} & \text{Y} & \text{YS} & \text{A} & \text{AL} & \text{AS} & \text{N} & \text{NL} & \text{NS} \\ \hline Black & 2 & 3 & 7 & 11 & 14 & 15 & 16 & 21 & 22 & 22 \\ Blue & 3 & 4 & 9 & 11 & 16 & 17 & 18 & 23 & 23 & 23 \\ Brass & 1 & 1 & 6 & 10 & 13 & 14 & 15 & 20 & 20 & 21 \\ Bronze & 2 & 3 & 8 & 10 & 15 & 17 & 19 & 22 & 23 & 23 \\ Copper & 1 & 2 & 7 & 10 & 14 & 14 & 16 & 21 & 21 & 21 \\ Gold & 3 & 5 & 10 & 13 & 17 & 19 & 20 & 24 & 24 & 24 \\ Green & 2 & 3 & 8 & 10 & 15 & 16 & 18 & 22 & 21 & 22 \\ Red & 4 & 6 & 10 & 13 & 17 & 19 & 20 & 24 & 25 & 25 \\ Silver & 2 & 4 & 9 & 12 & 16 & 17 & 18 & 23 & 23 & 23 \\ White & 2 & 2 & 6 & 9 & 13 & 15 & 17 & 20 & 20 & 20 \\ \hline \end{array}

As for the formula I used, it's difficult to transcribe as it frequently points to the lookup table "Monster Statistics By Challenge Rating" on page 274 of the Dungeon Master's Guide, and also the "Effective Hit Points Based on Resistances and Immunities" lookup table from page 277 of the same. I'll do my best to represent it here:

It starts by calculating [EHP] Effective Hit Points from the base monster's [HP] hit points, [CR] challenge rating, and type of "Significant Defense". A monster can have a significant defense type of [N] "None", [R] "Resistance", or [I] "Immune" as seen in the lookup table on 277. Since Shadow Dragons gain an irregular suite of resistances, I added a fourth, less potent option of [S] "Semi-Resistance" specifically for them. SD will equal a modifier between 1 and 2.

\begin{array}{r|lllll} \text{SD by CR} & \text{N} & \text{S} & \text{R} & \text{I} \\ \hline 1-4 & 1 & 1.5 & 2 & 2 \\ 5-10 & 1 & 1.25 & 1.5 & 2 \\ 11-16 & 1 & 1 & 1.25 & 1.5 \\ 17+ & 1 & 1 & 1 & 1.25 \\ \end{array}

$$ \text{EHP} = \text{Ceiling} (\text{HP} \times \text{SD}) $$

[EDCR] Effective Defensive Challenge Rating is a modified base CR found by looking up the EHP value under Hit Points on the 274 table and returning the listed CR.

Next you calculate the [DCR] Defensive Challenge Rating from the monster's EDCR, [AC] base Armor Class, and [EAC] Expected Armor Class. EAC is found by looking up the EDCR value as CR on 274 and returning the listed Armor Class. For every two points the AC is over the EAC, it should increment the DCR by 1.

$$ \text{DCR} = \text{EDCR} + \text{Floor}\left(\frac{\text{AC} - \text{EAC}}{2}\right) $$

Next you need to calculate the monster's [DPR] Damage Per Round, which should be the maximum damage the monster can deal in a turn. Assume that a small area of effect attack will hit two targets, and a large one will hit three. Since a dragons breath weapon can't be used every turn and has a 1/3 chance of recharging, calculate the the [DBW] Damage of the Breath Weapon and the [DOA] Damage of Other Attacks separately.

$$ \text{DPR} = \frac{\text{DBW} + \text{DOA} + \text{DOA}}{3} $$

[EOCR] Effective Offensive Challenge Rating is a modified base CR found by looking up the DPR value as Damage/Round on 274 and return the listed CR.

Next you calculate the [OCR] Offensive Challenge Rating from the monster's EOCR, [AB] base Attack Bonus, and [EAB] Expected Attack Bonus. EAB is found by looking up the EOCR value as CR on 274 and returning the listed Attack Bonus. For every two points the AB is over the EAB, it should increment the OCR by 1.

$$ \text{OCR} = \text{EOCR} + \text{Floor}\left(\frac{\text{AB} - \text{EAB}}{2}\right) $$

The final Challenge Rating is the rounded average of DCR and ACR.

$$ \text{CR} = \text{Round}\left(\frac{\text{DCR} + \text{OCR}}{2}\right) $$

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You may now see why I didn't post the "formula" the first time, as it five times references lookup tables (six times, if you include finding the base CR from HP). Plus, it took me six hours to get it all together and working and I was too burnt out to "show my work". I hope this was helpful to someone.

Answer 2

Use the DMG's guidance on modifying monsters

The DMG, on pages 273 through 279, includes guidance on modifying monsters and creating new monsters, including guidelines on how to calculate a monster's effective CR. This involves breaking down the monster's capabilities into a "defensive" challenge rating and an "offensive" challenge rating, then averaging the two.

Young red shadow dragons

A young red dragon has a normal CR of 10. By following the DMG's guidance on calculating its defensive CR, we see that:

• It has 178hp, which looks up to CR 8 in the table
• It has an AC of 18, 2 points higher than the expected AC of 16 for a CR 8 creature, so bumps CR to 9
• It doesn't have lots of damage resistances or immunities, so we don't treat it as having more HP for these calculations

For the DCR and OCR to average to 10, I'll assume the offensive CR is 11.

The single biggest difference adding the Shadow Dragon template to the dragon makes is to add lots of resistances that make it very hard to hurt (maybe only resistances it has in certain conditions... but then why would it ever stick around to fight you if it wasn't in advantageous terrain?) By adding so many resistances we justify raising the dragon's effective HP for the purpose of calculating its CR, as per Step 9 of the guidance on page 277:

• 178hp is CR 8
• Multiply HP by x1.5, for a CR 9 creature with many resistances, to give ~267hp
• 267hp is CR 14 (and AC 18 is appropriate for a CR 14 creature)

Averaging DCR 14 with an OCR of 11 gives us CR 13. It's plausible that the dragon's OCR was also given a slight bump because of the way its breath weapon can instantly kill adventurers now (the changes to damage type wouldn't normally make any change to its CR), which does make it meaningfully more lethal than the base creature - but OCR 12 would also average to CR 13 here.

At any rate, the change to the young red dragon's CR when applying the shadow dragon template as given in the Monster Manual seems consistent with the DMG's guidance on calculating CR when modifying/creating monsters.

So what of the Black Dragon Wyrmling?

This is a little harder to calculate since the wyrmling's stats are at the very low end of the scale and the DMG isn't super clear about whether or not CR changes should be steps on the table or full points, but we'll give it a go:

A Shadow Black Dragon Wyrmling's defensive CR:

• 33hp maps to CR 1/8
• AC 17 is 4 points higher so bumps CR two points to 2 (2 1/8) or 1/2 (two steps up from 1/8)
• For a CR <4 creature with many resistances, multiply HP by x2, so 66hp
• 66hp maps to... CR 1/2, again bumped by the higher AC by two points - to 3 (2 1/2) or 2 (two steps up)

And offensively:

• ~21 damage per round (assuming one use of breath weapon hitting two targets and two bite attacks over a three round period, as per DMG's guidance) maps to CR 3
• +4 to hit is appropriate for CR 3 so no change

Averaging those gives us CR 3. (Note that the base creature could be CR 3 or CR 2 depending on how we calculated it - as the MM gives it CR 2, that suggests the second method of stepping on the table rather than adding entire points is the better one.)

Fact is, the shadow dragon template doesn't make the black dragon wyrmling effectively that much more durable - it's maybe twice as hard to kill as it was before on paper, but against a normal adventuring party that change in effective hit points would probably only give it one more round of life.

The change to the breath weapon is probably the most significant factor in this case, since at the low levels where a CR 2 or 3 creature is an appropriate challenge, 5d8 damage could easily one shot a character and then they're dead outright. Bumping the OCR by a point to account for this gives us DCR 3 and OCR 4 which averages to 4 (3 1/2) - so 3 to 4 would seem a reasonable CR for the shadow black dragon wyrmling.

Of course, the DMG is quick to point out that the guidelines it provides are just that, and you may need to adjust the CR of a creature if it proves to be more or less dangerous in actual play. Many monsters in the published material have challenge ratings which are slightly off from what you would calculate from the DMG's guidance because of their special abilities and features that change the effective challenge they provide.

Answer 3

The process is actually pretty straight forward and follows the official rules for CR calculation

First of all, sorry for posting an answer to such an old question, but after trying to puzzle out the Young Red Shadow Dragon's CR myself just now and seeing that none of the answers here actually calculate it the correct way, I felt the need to share my findings for anyone wanting to use that template in the future.

I admire the work the other answers* have done, such as inventing a Semi-Resistance category for calculating the effective HP or doing calculations on the Monster's experience points, but I want to tell everyone who wants to use the Shadow Dragon template and calculate its Challenge Rating: You do not have to do any of these. Calculating a Shadow Dragon's new CR, as exemplified with the Young Red Shadow Dragon in the Monster Manual, follows the official rules for determining a Monster's CR to a T, without needing anything else. These rules can be found in the DMG under the "Creating a Monster" Section starting on page 273.

As mentioned by you, a Shadow Dragon gains a lot of new abilities and gets changes to existing ones: Resistance to Necrotic damage, Expertise in Stealth, Necrotic Damage on its Bite and Breath and the fact that its breath now instantly kills and creates Shadows. However none of the above change its CR, the only ones that do are the resistances it gets through Living Shadow** and its newly acquired Shadow Stealth. The latter of which is listed on Page 281 of the DMG as one of the Monster Features that effects a creatures CR.

So to answer your question directly, to determine a Shadow Dragon's CR you just need to calculate the CR of the regular Dragon using the guidelines in the DMG but:

• Increase its effective AC by 4 because of Shadow Stealth
• Increase its effective HP because of the resistances from Living Shadow**

However, after that you will possibly have to recalculate the CR one more time, as these changes will often times increase the Dragon's CR so much that one or more of the following changes have happened:

• Its proficiency bonus has increased. This increases its attack modifier and saving throw DC and with that possibly its offensive CR
• Its CR is now over 10 and no longer requires you to add +2 to the effective AC for being able to fly and attack at range***. This decreases its effective AC and with that most likely its defensive CR
• Its CR has now crossed one of the thresholds for the Resistances Multiplier in the Table on Page 277 of the DMG. This decreases its effective HP and with that most likely its defensive CR

In the first calculation you will be using the regular Dragon's CR as the expected CR, while in the recalculation you will be using the CR you got as a result from the first calculation. I am certain this is the way they must have done it with the Young Red Shadow Dragon and therefore is the way we must do it, if we want to calculate the CR of a different Shadow Dragon for ourselves.

Calculating the Young Red Shadow Dragon

I am now going to demonstrate this process with the Young Red Shadow Dragon from the Monster Manual****. Carcer in his answer* tried as well to calculate the CR of that Dragon with the DMG's guidelines, ending up with the correct result CR of 13. However I believe this to be a a classic case of getting the right answer in the wrong way (as I am sure all of us have done in a Math Test at some point in our lives). The way I see it, there are at least five mistakes in his calculation, that I am going to correct in mine:

1. He didn't consider the Saving Proficiencies
2. He didn't adjust for being able to fly and deal damage at range***
3. He didn't calculate the OCR and just guessed it to be 11 (which it is not)
4. He didn't account for Shadow Stealth, as explained above
5. He didn't recalculate in the end

First Calculation with expected CR 10

To calculate the total CR we need to calculate the offensive and defensive CRs first, called OCR and DCR for short. For the OCR we need its average damage over three rounds. Which will be using its Breath once (as it recharges on a 5-6) and hitting two targets with it and then on the next two rounds using its Multi Attack giving as a total we then have to divide by three to get the average per round. This results in this equation:

(56 * 2 [Breath hitting twice] + (17 + 3.5 [Bite] + (13 [Claw] * 2)) * 2) / 3

The result is 68.3 repeating or 68 rounded down. Giving us an initial OCR of 10, which does not change if we look at its Save DC of 17 as the regular Save DC for CR 10 is 16 and it only changes at a difference of 2 or more points. Note that we are using the Save DC here as more than half of its damage comes from its breath weapon.

With the OCR calculated we now look at its DCR, which we get by taking its 178 hit points and multiplying them by 1.5 for the resistances** giving us 267 effective HP and an initial DCR of 14. The DCR however is then increased further to 18, because the expected AC here is 18, while the Young Red Shadow Dragon's effective AC is 26. 18 from the actual AC + 2 for flying and dealing damage at range***, then + 2 for having four Saving Throw Proficiencies and finally the + 4 from Shadow Stealth.

So the first calculation gives us a OCR of 10 and a DCR of 18 which gives us a CR of 14 for the Young Red Shadow Dragon. We know this isn't correct, which is why we now have to do the recalculation.

Second Calculation with expected CR 14

For the second calculation the Damage Output does not change, still giving as an OCR of 10, however since its Proficiency Bonus increased we now have a Saving Throw DC of 18 (as seen in the finished Stat Block) which increases this OCR further to 11.

The DCR starts off with the 178 HP as before, but instead of multiplying by 1.5 for the resistances** we only multiply by 1.25, because of our new expected CR. This gives us only 222 effective HP and an initial DCR of 11. This is increased to a DCR of 14 however, because of our 24 effective AC, as opposed to the 17 which would be normal for a DCR 11. The Dragon's effective AC is now two points lower as it no longer includes the +2 Bonus for flying and doing damage at range, which is only for expected CRs of 10 or lower.***

So we end up with an OCR of 11 and a DCR of 14. These give us an average of 12.5 and its now up to the Statblock creator to decide whether it should be CR 12 or 13. WotC decided for the latter and I suspect this to be because of the Shadow Dragon's new Instakill-Shadowmaking-Breath, which as discussed doesn't directly change the CR, but can for sure be used as a tie breaker in such a scenario.

Calculating the Black Shadow Dragon Wyrmling

And now that we have shown the method works for the example Shadow Dragon Statblock from the Monster Manual, we can use it to create the CR for your Black Shadow Dragon Wyrmling.

First (and only) Calculation with expected CR 2

Its OCR is one Breath hitting twice + 2 bites over the first three rounds and then taking the average which is:

(22.5 * 2 [Breath] + (7.5 + 2.5 [Bite]) * 2) / 3

Resulting in 21.6 repeating which is rounded up to 22. This gives an initial OCR of 3, which is decreased to OCR 2, because of it only having a DC 11 for its breath.

Then its DCR is initially ½, from having 33 HP and doubling that to 66 because of the resistances**, but is then increased by 6(!) Stages to DCR 6 from having an effective AC of 25, which is 12 points greater than the expected AC of 13 for DCR ½. The effective AC comes from the 17 actual AC + 2 for flying with ranged attacks*** + 2 from having saving proficiencies and + 4 from Shadow Stealth.

So we end up with an OCR of 2, a DCR of 6 and thus an average, final CR of 4, for the Black Shadow Dragon Wyrmling. Which we do not have to recalculate, as its proficiency bonus has not changed and we also did not cross any of the HP thresholds.

To Summarize

Calculating a Shadow Dragon's challenge rating is a straight forward process that requires neither guesswork nor secret techniques, just the knowledge of how to calculate CRs in general. It just appears this conclusion might have successfully hidden from us for the last five years, like a Shadow Dragon in dim light.

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*To clarify: My references to the other answers are not in any way meant to downplay these users or their work. The purpose of these references is to show that none of these are actually the right answer to the question, as none of them do the CR calculation the proper way, which is the reason I am posting an answer to this question 5 years after it has been asked.

**As pointed out correctly by Carcer it is safe to assume the Shadow Dragon will always be fought in dim light or darkness and therefore these resistances will need to be included in the CR calculation.

***As of now I am actually not entirely certain whether the CR of the Dragon's were calculated with the +2 AC from flying and dealing damage at range, which can be found under Step 14 on Page 279 of the DMG. Even though they literally can do this, they can only do so in one of the first three rounds of combat, and I do not know if this fits the requirement. I test calculated a few of the wyrmlings and their CRs where all correct whether or not this bonus was included. For the sake of consistency this post assumes that this AC bonus needs to be included. However I am open to anyone who knows the answer to this and might also research this topic further and adjust this post in accordance to what I learn.

****I could not have done this without the help of this Question that taught me how to correctly calculate the CR of the regular Young Red Dragon.

Answer 4

Use relative XP

As was indicated in the question, (MM, 84):

When a dragon becomes a shadow dragon, it retains its statistics except as described below.

Challenge Rating is not listed as one of the exceptions.

A strict reading would indicate that you keep the same CR when the dragon transforms. The Monster Manual describes what a statistic is on page 6:

A monster's statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster.

Under the broad heading of Statistics you will find many different stats including Challenge Rating (MM 9, under sub-heading Challenge).

So the rules indicate the CR stays the same, however the authors have provided a template which increases the CR of the Young Red Dragon from 10 to 13 after it was transformed to a Young Red Shadow Dragon.

Why the discrepancy?

I don't know. But it's there. More importantly, if it's an oversight, where did the authors make the error? Is the error in not including CR as an exception, or is the error in changing the CR in the template? Or maybe there is no error and I'm missing something (?).

Considering that CR tells you how great a threat the monster is, (MM 9) we need to determine if the stats gained by the Young Red Shadow Dragon make it more formidable than the standard Young Red Dragon. Once we determine this we can know where the error is in the text.

What makes the Shadow Dragon more formidable than it's former self?:

• If the Shadow Dragon is in dim light or darkness it has resistance to all damage types except force, radiant or psychic. That's a huge benefit.
• With its hide abiity as a bonus action in dim light or darkness, and extra stealth abilities (+8) it is far more likely the Shadow Dragon will surprise a party of adventurers.
• The Shadow Dragon's breath weapon instantly kills a combatant who falls to 0 HP, removing the death saving throw mechanic.
• And an Undead Shadow (MM 269) immediately rises from that dead combatant's corpse to do the dragon's bidding.

The only negative impact is that the Shadow Dragon is at disadvantage on attack rolls if it is in sunlight.

The Young Red Shadow Dragon is more formidable than the Young Red Dragon.

It seems that the error is in not including the CR in the list of exceptions. Is the Young Red Shadow Dragon deserving of +3 CR? Yes, this seems about right, however as a general rule I would not add +3 CR to a Shadow Dragon. A more accurate adjustment would be to take the relative XP increase from the template and apply the CR based on the XP increase.

Doing the math.

The standard Young Red Dragon has an XP of 5,900, while the Young Red Shadow Dragon is indicated as having an XP of 10,000. The XP has increased by a factor of 1.694915, that's 10,000/5,900. I'd call it 1.7. Apply that to the 450 XP of the Black Dragon Wyrmling and you get 450 x 1.7 = 765. Looking at the XP/CR chart at the front of the MM shows us that 765 XP is a CR of 3.

The Black Shadow Dragon Wyrmling would have a CR of 3.

I can guarantee you though: that beast is staying in its lair and never going out in sunlight without SPF I don't think so.

One final point is that according to its description, The Shadow dragon might retain or lose any or all of its lair actions or inherit new ones, as the DM sees fit. This could also increase or decrease the CR and would depend on the changes made. Of course this only applies to Dragons that already have a lair action (or does it? But I'll leave that for another question).