Does Spell Resistance scale when advancing a monster?

Question

I just advanced a Canoloth (MM 3, p. 200) as I wanted to use it against a level 14 party. It is originally CR 5, and I wanted to raise it to CR 8. So I added 4 HD (CR+2) and the elite array (CR+1). This raises a lot of stats, especially saves, but against a level 14 party, Spell Resistance 18 is miserable. Effectively, the Canoloth lost half its Spell Resistance compared to a basic Canoloth matched against a level 11 party.

So would you judge that raising SR to 21 would raise the advanced Canoloth's CR above 8? Or should this just be part of the advancement to CR 8?

Answer 1

As a general rule, spell resistance is 11+CR so you would simply add the additional challenge rating to the creature's spell resistance directly.

From the monster manual page 300 sidebar, BEHIND THE CURTAIN: SPELL RESISTANCE AND DAMAGE REDUCTION:

Spell Resistance: If you choose to give your monster this ability, you’ll probably want to set the resistance number equal to the creature’s CR + 11. This means that a character of a level equal to the creature’s will have a 50% chance to overcome the monster’s spell resistance (barring the Spell Penetration feat). For example, a 12th-level character has a 50% chance to overcome spell resistance 23, so 23 is a good spell resistance number for a CR 12 creature. You may need to adjust a creature’s spell resistance number after you finally settle on a CR for the creature. If you want a highly magic-resistant creature, set the monster’s spell resistance higher than CR + 11. For lesser resistance, set the spell resistance lower. For each point of difference, you’ll change the chance of successfully overcoming spell resistance by 5%. For example, a 12th-level caster has a 45% chance to overcome spell resistance 24, and no chance to overcome spell resistance 33.