How does a dragon gain Challenge Rating & thereby increase their spell power?

Question

According to the spell casting variant suggested in the Monster Manual in the True Dragon section, dragons gain a number of spells based on their charisma and their maximum level is based off of 1/3rd of their Challenge Rating. Here is a table that sums it up.

Assuming a dragon reverse extrapolated and is aware of this variant for draconic spell casting, they would seek to gain greater CR in order to cast the better spells. Take an Ancient Green with CR 22, she needs a few more CR 'points' so as to gain Clone or Mighty Fortress.

Note: ALL the questions below revolve around True Dragons gaining increased Challenge Rating. If I require a separate StackExchange question for each, please let me know!

• Does a dragon existing-residing 'in lair' gain CR? Does their maximum spell level 'drop' the moment they leave?

• Does a leading dragon gain CR based on the number and quality of her servants, minions, slaves, toadies, &/or henchpersons (or 'livestock-property' in the case of Green dragons)? Would this also increase based on magic items, traps or well-defended real estate (such as castles on mountains - they have a lot of hit points and good armour class, making a dragon MUCH tougher).

• If any True dragon becomes a Shadow dragon, how much does it gain in Challenge Rating? Why?

• Does a vast &/or terrifying amount of knowledge equate to CR? It would mean better tactics, strategy &/or knowing weaknesses of friends and foe alike. Does a vast amount of intelligence infrastructure (i.e. 'a powerful spy network & strike force') equate threat and danger and thus increase CR?

• Would gaining the spell power of an arch-mage (CR 12) add to her high-ranking CR? If so, how much?

Most dragons would want at least CR 24 for 8th level spells at the very least. Getting CR 27 would allow for the real game-changer spells like True Polymorph and Wish.

Long story short: Assuming a dragon figured out that CR = Spell power, how would such a being best gain Challenge Rating?

Answer 1

CR is a purely metagame construct

Remember that Challenge Rating is purely a construct of a game. Within the fiction, a dragon wouldn't even know that it existed, much less seek to increase it directly.

In fact, while all the things you listed are controlled by CR within the game, that simply serves to give the DM an easy way to quickly create the monster they want. Most GMs will start with a target challenge rating and then select or build the monster to fit that. The other way around certainly does happen, but in my experience it is far less common and sometimes requires fudging something else to make sure the PCs experience the right amount of challenge.

As Upper_Case helpfully pointed out, CR reflects abilities rather than the other way around. To the extent that the fiction and CR interact, a dragon's CR goes up when it gains new abilities rather than saying it has a certain CR and thus has certain abilities. Of course, as a meta-game construct it is likely often the case for a recurring dragon that many DMs will decide the CR needs to increase and therefore add the proper abilities from the MM, but that is a shortcut in a way. If a DM in a truly open world game wanted to create a dragon with a rich history and have it evolve as time goes on, the DM would decide what abilities it had according to the history then calculate the CR based on those. The DM would then increase the CR as the dragon gained new powers in fiction (or reduce the CR if the dragon somehow lost an ability of asset it used to have).

In fiction, a dragon will generally seek to increase its power the same way other beings doing.

Within the fiction, a dragon will generally seek to increase its power the way most other things will. It will seek to improve its wealthy and access to items, it will practice and develop the skills and abilities it finds useful, it will seek to amass followers. But it does these things to accrue power, not in some attempt to increase a challenge rating that it will have no concept of.

The one big exception is that a Dragon tends to grow more powerful with sheer age for most of its life with adult dragons being generally more powerful and larger than younger dragons and so on. To a degree, the dragon grows stronger by simply staying alive.

Answer 2

First, the general principle that answers most of these:

A monster's challenge rating is based solely on its own statistics

Does a leading dragon gain CR based on the number and quality of her servants?

Would gaining the spell power of an arch-mage (CR 12) add to her high-ranking CR? If so, how much?

No. If these servants or allies are involved in combat, their individual CR is considered when determining XP and encounter difficulty, but this is a property of the encounter, not the dragon herself.

Does a vast &/or terrifying amount of knowledge equate to CR?

In general, no. CR is not evaluated relative to a particular opponent. While being better informed might make the dragon better capable of taking down the foes her information concerns, it does not increase her CR. There is a potential objection to this point, which I will address a bit later.

If any True dragon becomes a Shadow dragon, how much does it gain in Challenge Rating? Why?

We know that the dragon's challenge rating does increase, due to the example shadow dragon template in the Monster Manual. The description does not explain how much the increase is, or why, but we can infer from the changes to the stat block that the reason is a combination of Shadow Stealth and Living Shadow.

Per DMG p.277, multiple resistances at this CR should give between a 1.25x and 1.5x multiplier to the final CR. Indeed, this is reflected in the jump from CR 10 from a young red dragon to CR 13 for a corresponding shadow dragon.

For a rough estimate, this is probably sufficient, and it's likely that Shadow Stealth doesn't provide a great enough change to make a difference.

Does a dragon existing-residing 'in lair' gain CR? Does their maximum spell level 'drop' the moment they leave?

Here's where things get tricky.

CR does not exist in the game

At risk of stating the obvious, challenge rating is a purely metagame concept. A dragon in its lair most likely gains CR (per the multiple other creatures with lair actions for which this is true,) but such an increase is not really an inherent change to the dragon itself.

You ask whether this increase would change what spells the dragon knows, and it should be clear that that outcome wouldn't make sense from a consistency standpoint.

Similarly, to re-address the knowledge question above: it's possible that a dragon is about to encounter an opponent for whom specific knowledge is necessary to even be a threat to them. In that case, that knowledge could change the dragon's damage output to the extreme, one could argue changing the CR calculations as a result.

Again, this outcome doesn't make sense. The dragon's preparedness for a fight shouldn't affect what spells it knows.

So in general:

A dragon's CR (for this purpose) should not depend on the situation it is in

We freely modify CR as a creature gains or loses combat abilities, but this variant rule is a mere approximation. In the fiction of the game, the dragon is learning spells because it is older and stronger, or because it's a more powerful dragon than its cousin of a different color.

Interpreting this rule too literally leads to very strange situations. For example, you might well ask if the dragon had a really nice suit of custom dragon armor, increasing its defenses, would that affect its CR. Of course, it would, from a calculation standpoint. But hiring a high quality armorer in order to learn Clone is... probably not the intent here.

If you're attempting to have a dragon aware of this concept in-game, you're applying in-game logic to something that's a purely mechanical construct. In doing so, you have free reign to use a framework that makes more sense than the literal.

In summary:

I'd recommend not considering lair actions, equipment, or foreknowledge in your calculations.