How to determine a creature's Spell Resistance

[An Enemy Spy]

I'm making an NPC that is a Movanic Deva and while the book gives them a SR of 19, I don't know if that holds true now that I've rerolled the ability scores. I can't find the rules that tell you how to know what a creature's SR is.

[Inevitability]

Changing ability scores should never change SR.

[pabelfly]

I haven't seen any creature or class with SR that scales off ability score, but 3e is a large game with a lot of rules. It's entirely possible that there's one creature or class with SR that scales off ability score and I'm not aware of it.

As for this specific Spell Resistance, it looks like it's just a flat SR 19, and doesn't scale.

[Beni-Kujaku]

In most cases, changing ability scores and adding templates does not change the SR, while increasing the racial hit dice of the creature does at the same rate as it increases the CR (examples: (Elder) Odopi, Mivilorn, explicitly stated in MM2). Adding class levels is inconsistent (in MM1, all creatures that are presented with and without class level explicitly say that their SR increase with their class levels, but that is not the case everywhere, for example the fiends in Fiend Folio do not increase in SR, but in MM4 the SR for the justice Archon is said to increase, but not by how much, as if +1 per class level would be the norm that we should always assume. In general, you can assume that the SR is not supposed to scale unless it says so, even if making it always scale with CR would be much better imo).

It's entirely possible that there's one creature or class with SR that scales off ability score and I'm not aware of it.

There is at least the Yuan-ti Cultist, who has SR scaling with Wisdom.

[Remuko]

Its spell resist, like far too many creatures is never explained. From eyeballing it, IMO, it's SR is equal to its CR + 10. The concept of any creature having static SR that doesn't scale makes no sense to me, so I always assume it scales with HD or CR unless explicitly said otherwise.

[Rynjin]

SR is usually based on Hit Dice CR to my knowledge. Pathfinder even gives some handy guidelines you might find useful, as it has a few SR "templates".

"Weak" SR is 5 + CR.

"Average" SR is 11 + CR

"Powerful" SR is 15 + CR.

"Static" SR is whatever the template/creature says it is. Like the Forsaken lich template (CR+2) that has a flat 25 SR.

These do not necessarily correlate to the power of the actual monster. Eg. an Ancient Red Dragon has SR 30 at Cr 19, because True Dragon get 11+CR in SR. This is similar to an Erinyes (CR8 with 19 SR).

"Weak" SR is usually limited to player races, like the Drow Noble. You get a little SR, but not very much. "Powerful" SR is extremely rare and limited to creatures who have their SR as one of their defining features, like the Grimm (which has SR 15+CR against Fey magic only, and 11+CR against everything else).

A Movanic Deva is not defined by its SR, and so would default to 11+CR...though this logic does appear to be different than 3.5's (if it has any) as the Deva in PF has an SR of 21 as you'd expect for a CR 10 creature in that system, while 3.5 has 19 at CR 9, or 10+CR.

Still, this should give you a decent guideline to work with going forward. Pick whether you like 10+CR or 11+CR better and have at it. If you raise the stats on a creature enough its CR SHOULD go up, similar to applying a template, so keep that in mind.

[Zanos]

SR is usually based on Hit Dice to my knowledge. Pathfinder even gives some handy guidelines you might find useful, as it has a few SR "templates".

3.5 has similar guidelines in the SR sidebar on page 300 of the Monster Manual.

[Rynjin]

Noticed I typed Hit Dice instead of CR there lol. Fixed.

[Saintheart]

With 3.5's rules on challenge being more rule of thumb than precise, it's the reason SR has a particular number that's important.

The design idea behind Spell Resistance is that SR should lolnope a PC's spell about 50% of the time where the PC's level and the creature's CR are the same. Absent, of course, the PC picking Spell Penetration or similar. You can see that on MM1 at p. 300 as Zanos has pointed out.

SR keys to CR because CR's designed to say to the DM: 'this single monster should make for a non-cakewalk, non-TPK, middle-of-the-road encounter for a party whose level matches the monster's CR.' WOTC assumes that a classic 4 man party should - on average - burn about 25% of daily resources killing a single monster whose CR matches their level. Remembering D&D is designed at least in part as a game of attrition, and SR is mainly designed to add to that attrition by fizzling spells.

Most of the time Caster Level is 'meant' to be equal to character level (assuming a straight-class caster who's not steroiding his Caster Level in one of the myriad ways pabelfly chronicles in this fantastic thread :) If a level 6 mage is rolling to beat SR against a CR 6 monster, the mage's Caster Level will likely be 6 and thus the Caster Level check 1d20+6. Thus the monster's SR normally should be 11+6 = 17, i.e. imposing around a 50/50 chance the mage's spell will futz out and not affect the big trundling lump of hitpoints.

There's no way you could lock SR to the number of HD a monster has, because virtually all monster types get multiple HD for every +1 to CR, and the number of HD varies depending on the type: Undead get 4 HD, Dragons get 2, and so on.

However, "add 1 SR for every 1 in CR" as an 'ironclad' rule as far as I can tell only appears in one place in MM2: on page 21, as part of an example of how to advance the Outsider mentioned there. It might be that this rule was really only meant to apply where you advanced a monster under WOTC's rules, not as a general rule for all NPCs, and maybe not even when you add class levels. The rest of the time, it's more MM1's rule of thumb and 'probablies' and 'maybes'.

We know there's plenty of monsters whose SR is fixed, and the SRD explicitly says you do not increase SR when advancing a monster unless the statblock specifically calls for it. That might be the designers thinking that scaling SR makes a creature much more powerful than necessary considering the other stuff the creature picks up in advancement.

Really when asking "What SR should this be if I advance it or give it class levels?", you should be asking a different question: "Do I want to lolnope the party's spells statistically 50% of the time when they face this particular critter, because that's what SR was at least designed to do?" If the answer's 'Yes', then 11 + CR is a fast Procrustean way of doing so. If the answer's "No, because the critter does a lot of other stuff ripping 25% from the party's daily-use resources," then SR maybe can be left untouched.

[glass]

Changing ability scores should never change SR.

Not directly perhaps, but if the ability scores increase enough to be worth +1 CR, that will usually be worth +1 SR.

[Chronos]

There might not be an explicit rule for it, but most creatures with spell resistance do seem to have it at approximately 10+CR, and as Saintheart explained, there are reasons for it to be at about that level. So I'd go with that.