SR and caster level for advanced creatures

[shilsen]

As the title says, how does one improve Spell Resistance and caster level for a creature advanced by hit dice?

For example, the basic bearded devil (MM, pg.52) has 6 HD, SR 17 and uses its greater teleport ability as a 12th lvl caster. If advanced to 8 HD (which, by the book, increases the CR by 1), does the SR and caster lvl change, and if so, but how much?

Thanks, in advance.

 

[CalrinAlshaw]

To me it seems ya kinda have to look at what formula's they are using.
SR17 seems like 11+HD.
and CL 12 seems like HDx2

Calrin Alshaw

 

[shilsen]

To me it seems ya kinda have to look at what formula's they are using.
SR17 seems like 11+HD.
and CL 12 seems like HDx2

Calrin Alshaw

That's what I was originally planning to do, but I figured it might skew the CR a bit much if advancing by hit dice a lot.

The basic barbazu is CR5, with 6 HD, SR 17 and CL 12. If advanced to 18 HD, it should be about CR 12 (+6 for 12 outsider HD and +1 for large size). SR of 11+HD makes it 29. Darn high for a CR12 critter (higher than that of any devil in the MM other than a pit fiend and than any demon at all, including the balor). I think I'll just eyeball it and make it SR = 12+CR.

Wait a minute - going through the MM, I see that is the exac formula that seems to have been applied to the devils: SR = 12+CR (Barbed Devil - CR 11, SR 23; Bone Devil - CR 9, SR 21; Chain Devil - CR 6, SR 18, and so on). And demons get SR=8+CR (except the succubus, which is CR7, SR 18). Interesting.

 

[the Jester]

By the rules as written, I don't believe caster level or SR increase. However, I am a firm believer in tweaking advanced monsters to make sure that they actually challenge the party (sometimes much easier than others).

 

[Garnfellow]

By the rules as written, I don't believe caster level or SR increase. However, I am a firm believer in tweaking advanced monsters to make sure that they actually challenge the party (sometimes much easier than others).

I agree that the general rules for monster advancement are strangely silently on this question*. A few individual monster entries like drow specifically mention that caster level and spell resistance increase with additional class levels or HD; if the individual monster entry does not specifically state otherwise, advancing monsters does not affect caster level or SR -- although I also agree with Jester that DMs should feel free to tweak out advanced monsters to suit their needs**. I often add +1 caster level/SR for every new +1 the advanced monster's Challenge Rating.

* In the example of advancing a monster in the Monster Manual 2, the Spell Resistance is explicitly noted as increasing by +1 for every +1 CR. However, because this rule has not been explicitly stated elsewhere, and was never incorporated into 3.5, I'm not sure it can be considered "official," if that means anything anyway.

** Sean K. Reynold's excellent sourcebook Anger of Angels has several feats available to fiends and angels that allow them to pick up/improve spell resistance, energy resistance, and damage reduction. I've found these feats very useful in tricking out individual fiends like your advanced bearded devil.