I am creating the Big Bad Evil Guy for my campaign and I don’t know what the CR would be.
BBEG
Medium humanoid , lawful evil
Armor Class 22 (Natural Armor)
Hit Points 285 (2d8/18d6+200)
Speed 30 ft.\begin{array}{cccccc}
\text{STR} & \text{DEX} & \text{CON} & \text{INT} & \text{WIS} & \text{CHA} \\
10\;(+0) & 13\;(+1) & 30\;(+10) & 30\;(+10) & 14\;(+2) & 16\;(+3)
\end{array}Saving Throws Dex +7, Con +16, Int +16
Skills Athletics +6, Deception +15, Insight +8, Intimidation +9, Persuasion +15, Religion +16
Damage Resistance magical damage
Senses passive Perception 12
This is a Rogue 2 / Wizard 18 PC-turned-NPC bad guy. He has a spell attack bonus of +16, and a spell save DC of 24. His stats are granted by magic items. He knows all magic spells available in D&D 5e and has most of the known magic items in the game.
I can’t figure out what the CR would be. Can anyone help? What is his CR?
Best Answer
Impossible to tell; it depends on his preparation
The Dungeon Master's Guide has rules on determining the challenge rating of creatures, and they depend on factors such as survivability and damage per round. You mention that this wizard "knows all spells" and "has most known magic items in the game".
Unless you've also decided that this guy can also break the rules for item attunement (he's already got stats and natural armor far higher than normal humanoids), that means his items could be any combination of 3 items from the DMG that require attunement, and any other number that does not. (Thanks to Miniman for pointing this out in a comment.)
His challenge rating will depend entirely on what spells he has prepared and what magic items he is using at that moment.
If all the spells he has prepared are utility and survival spells, and none of the magical items he is bringing along allow him to do damage, his challenge rating is essentially negligible, he'll be a huge bag of HP and weirdly high natural armor. If however he has some strong combat spells prepared and some powerful magic items attuned, he'll be able to single-handedly kill ancient dragons.
In other words, we can't tell you what the challenge rating of this creature is, because it depends on circumstances. If you want to figure it out, you'll first need to determine what spells he'll have prepared and what magic items from his vast collection he'll be using, then you can determine a challenge rating. If, in a later encounter, he shows up again with different spells and different items, he'll have a different challenge rating.