[RPG] How to roll against Spell Resistance

RAW, the minimum CR is 1

Dealing with CR is always at least a little bit iffy, and it's more a math game than a way of determining how special or interesting a creature is. That is to say, it's less that a creature gets Legendary Resistance because it's interesting but more that it's interesting because it has Legendary Resistance. More subjective thoughts on whether or not a creature is cool or interesting do not factor into CR.

The DMG has guidance on how Legendary Resistance affects CR

For determining whether or not a homebrewed creature can or should have this feature, the DMG suggests considering Legendary Resistance to add effective HP to a creature, based on its CR without the feature, for each per-day use:

Each per-day use of this trait increases the monster’s effective hit points based on the expected challenge rating: 1–4, 10 hp; 5–10, 20 hp; 11 or higher, 30 hp. (DMG, Chapter 9: Dungeon Master's Workshop, Creating a Monster, NPC Stat Blocks, Monster Features Table, Legendary Resistance)

So you can give this to any creature you like, but it will potentially alter the CR to be greater than you may prefer. This prescribed method allows you to estimate that impact (to the degree you can ever reliably determine CR).

That the guidance has a value for CR 1 creatures definitively demonstrates that the minimum CR the rules define as valid for attaching Legendary Resistance to is 1. But can a creature that actually has Legendary Resistance available three times per day have a CR of 1?

An example by the numbers

Let's look at a pretty straightforward example by modifying a creature that is already CR 1 to include Legendary Resistance (3 per day) to see how the CR changes. We'll use a CR 1 creature because the guidance does not have any suggestions for challenge ratings below 1. We will also use the "Modifying a Monster" guidelines given in the DMG (Chapter 9: Dungeon Master's Workshop, Creating a Monster, Modifying a Monster), especially the "Monster Statistics by Challenge Rating" table.

A Direwolf is a CR 1 creature as written:

1. Defensive Rating

   • It has an average of 37 hit points
   • It has an AC of 14, one point higher than the table suggests for that number of hit points. This is less than the two points of additional AC which would cause us to increase its Defensive Rating, so we do not modify the Defensive Rating further
   • The final Defensive Rating is 1/4

2. Offensive Rating

   • It deals an average of 10 damage per round, which corresponds on the table to a suggested attack bonus of +3

   • The Direwolf also has the Pack Tactics feature, which the Monster Features table suggests should raise its effective attack bonus by 1

   • The Direwolf's actual attack bonus is +5, which is only 1 point greater than the effective +4 indicated by the previous two bullets and so the Offensive Rating does not need further modification

   • The Offensive Rating is 1

3. Average Challenge Rating

   The next step is to take the average of the Defensive Rating and Offensive Rating, which is ((1/4) + 1) / 2 = 0.625. We are then instructed to round up to the nearest CR, which in this case is 1. So the listed rating of CR 1 checks out according to the "Modify a Monster" rules.

A Direwolf with three uses of Legendary Resistance per day

Now let's add three uses of Legendary Resistance per day. The guidance suggests that this should count as an additional 30 effective hit points for a CR 1 creature, giving us an effective total of 67 hit points.

This moves us one row further down the Monster Statistics by Challenge Rating table, from a 1/4 Defensive Rating to 1/2, but that row still has a suggested AC of 13.

None of its other statistics have changed, and so the formula for Average Challenge Rating is (0.5 + 1) / 2 = 0.75. This also rounds up to the next CR, and the next CR is 1, just as before.

By following the "Modify a Monster" rules, we start with a CR 1 creature (the minimum for which the DMG offers guidance), give it three uses of Legendary Resistance per day, recalculate its CR, and end up with a creature that still has a CR of 1. Therefore the minimum CR for a creature which has that feature is 1.