So, a little help with Spellcasting and CR for monsters.

[SetArk]

Hail everyone!
So, i asked on the RAW Rules Thread, and got confirmation that, adding spellcaster special trait, to npcs, doesn't have a real ruling of impact on the CR.
So, i would ask you guys, how to you homebrew npcs/monster spellcasters?
How do you make calculations on the CR for then?

[mephnick]

Honestly all you can do is try and guess what effects the spells you select will have on the monster and recalculate their CR from there.

If it's likely going to use its blight spell on turn one, factor that into its offensive rating like you would any other attack. If it'll pop mage armour at the beginning of the fight add that to the defensive rating.

I find it a lot easier to limit NPCs/Monsters to a few spells you think are central to the role you want them to play and recalculate the CR from there. Giving monsters 30 spells like a PC and then trying to figure out their new challenge rating is a waste of time.

[SharkForce]

yeah, you're pretty much going to need to make up your own rules from scratch by looking at what spells you want to give the monster, based on your best guess regarding the offensive or defensive value (in damage, AC, HP, increased chance to hit, etc) that you can make. and even though the monster rules don't say a danged thing about it, changing the spells in question should *very* much have an impact on CR.

[MaxWilson]

Hail everyone!
So, i asked on the RAW Rules Thread, and got confirmation that, adding spellcaster special trait, to npcs, doesn't have a real ruling of impact on the CR.
So, i would ask you guys, how to you homebrew npcs/monster spellcasters?
How do you make calculations on the CR for then?

Since the only thing CR does is control how much XP the players receive, I don't mind eyeballing it, and I try to err on the side of generosity. There really aren't any guidelines for how much more dangerous an adult red dragon becomes when he's also a dragon spec 9 with access to Shield and Quickened Hold Person V, so if I take half the sorc level and add it to his CR for a total of CR 21, it's not like my players are going to complain that they got too much XP according to the DMG.

If you're asking for an evaluation of what spellcasting does to their danger factor, the answer is "spell casting dragons are exponentially more dangerous than regular vanilla dragons." (Blindsight + Darkness) + Shield + Dimension Door + Counterspell... That's an enormous force multiplier, especially in Combat As War, compared to a dragon who flies in a predictable pattern and tries to breathe fire on you at close range.