Are there guidelines for creating a lower-level Legendary Creature?

Question

A recent Wizard’s article, Unearthed Arcana: Encounter Building, states:

If you want to use a single monster, you’re typically best off using a legendary monster, all of which are specifically designed to make life interesting for a group.

The article identifies creatures of CR 1 as being an appropriate “Solo Monsters” for four first-level characters, for example.

I like the article’s advice, but the legendary creatures in the Monster Manual are mostly at least CR 10. There are no good examples of balanced, low-level “legendary” creatures, for solo fights.

What the core rulebooks say

Checking the DM Guide rules for Creating a Monster (pp. 273–283) I don’t see guidelines for customizing a creature to be legendary, or for creating one from scratch. And the Monster Manual section on Legendary Creatures (p. 11) describes legendary actions or lair actions, but does not provide guidance on designing your own.

Any Good Solution is Welcome

If there are guidelines from Wizards somewhere that I’ve missed, please post that link. Play-tested house rules would also be great. I have seen a third-party article or two out there — if folks have play-tested these suggestions, your impressions on them could also be a good answer.

Answer 1

The limits of the action economy (1 movement, 1 action, 1 bonus action and 1 reaction per creature per round) mean that a single creature is always at a disadvantage in terms of the number of things that it can do with respect to a group of creatures (like a PC party). The mechanical purpose of legendary actions (lair or otherwise) is allow the creature to break these constraints. In effect, legendary actions are simply things that a creature can do out of sequence.

In terms of determining a creature's challenge rating, the fact that an action is a legendary one is irrelevant: it is just something that needs to be included with all its other actions/bonus actions when determining its damage output (for offensive CR) or effective AC (for defensive CR). For example, an Ancient White Dragon has a damage output of:

• Multiattack: Bite/claw/claw (2 rounds in 3) for 19 + 15 + 15 = 49
• Breath Weapon: (1 round in 3) for 72 x 2 targets (probably) = 144
• Legendary: Either 3 tail attacks for 17 x 3 = 51 OR 1 tail and 1 wing for 17 + 15 x 2 targets (probably) = 49
• Lair: Two options that each do 10 damage to everyone in a huge area for 10 * 4 targets (probably) = 40

This averages out to \$ (49 \times \frac23) + (144 \times \frac13) + 51 + 40 = 175 \$ for an offensive CR of 22. Yes, I know it has a CR20 in the book - this is just one of the many CR anomalies in the Monster Manual.

In general, legendary actions are not needed for monsters with a CR of below 10 or so because the PCs cannot generally fully utilize the action economy themselves when these CR are suitable solo opponents. The problem that legendary actions were created to resolve doesn't really kick in until the PCs are getting an action, a bonus action and a reaction almost every round. If you want you can add legendary actions to low CR monsters and boost their CR a bit but its not really worthwhile.

As an aside, Legendary Resistance despite its name is not a legendary action.

Answer 2

The DMG has these guidelines

In page 278:

Overall Damage Output. ... When calculating a monster's damage output, also account for special off-turn damage-dealing features, such as auras, reactions, legendary actions, or lair actions.

In this case, only damaging Legendary Actions and Lair Actions really add to the Overall Damage Output. Actions that trip, paralyze, etc. aren't factored in.

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Page 280 also shows the amount of HP that each use of Legendary Resistance adds to your Effective Hit Points. No matter what you're using Legendary Resistance on, whether it's to avoid that Fireball or the Dominate Person or even Faerie Fire (too stronk!), it always increases your Effective Hit Points.

With these pages of the DMG, I was able to create CR 6 Legendary Creatures without a problem.

For example, here's a CR 7 one we playtested in the Backroom: First playtest, Second playtest. Here's a CR12 one we also playtested. Here's a CR7 full statblock of a boss monster I intended to be CR6 only, but it fared ok against 4 3rd-level PCs.

Answer 3

If you want to use a single monster, you’re typically best off using a legendary monster, all of which are specifically designed to make life interesting for a group.

To understand the context of this quote, I think it's important to first analyze some key features of "Legendary Monsters".

1. Legendary Resistance: the core premise here being the fact that your creature won't die to the first Hideous Laughter or Hold Person you see. This helps extend the fight and exhaust player resources.
2. Legendary Actions: this allows the single monster to act as multiple monsters because it can act several times / turn. It still has the same HP total, but the extra actions enable it to do more damage or affect more PCs each round.
3. Lair Actions: again this is a single creature acting as multiple. However, the Lair Actions only happen in a specific place, so you can make a boss harder to kill "at home".

If you're building a low-level boss, you probably want to take one or two of these, but give them in small doses. So while Dragon might have 3 doses of Legendary Resistance, you CR 2 boss would have 1. Likewise with Legendary Actions, give it 1 dose instead of 3.

Doing this will prevent a random Hold Person spell from ending the fight on round 1. It will also enable the villain to act a few times before the fight is over.

Note that you don't have to do these things for low levels. Often, just picking an appropriate monster and adding minions is enough to challenge PCs from level 1 to 4. Or adding spells to a monster. A group of 4 Kobolds + a single Kobold spellcaster with Burning Hands is enough to threaten a first level party. That stated, the fight could still be over in 2 rounds, so you won't see a lot of Burning Hands.

But if you want to make memorable bosses, then adding the Legendary features helps extend the fight a little and helps to make it more memorable.