[RPG] Are there guidelines for creating a lower-level Legendary Creature

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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.

Best Answer

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.

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