I'm working on a custom monster right now and I want to give it a Petrification Ray, as seen on the Beholder, as an action:
Petrification Ray. The targeted creature must make a DC (8+Spellcasting Ability+Proficiency Bonus) Dexterity saving throw. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.
I understand that each modification to a creature has a certain impact on CR based on what that ability will actually do in terms of gameplay (as seen on DMG280-281), but I can't figure out whether it increases defensive CR, offensive CR, or both—or by how much.
I apologize that this is a little open-ended, but since there's a formula behind CR calculation, there must be a true and objective answer to this question. (After all, the designers must have accounted for that numerical value when writing the Beholder.) Does anybody know what the value in question is, or how to find it?
Best Answer
Let's consider a different question for a moment. Suppose I'm building a monster, and I give it a melee greatsword attack with +4 to hit and 2d6+4 damage. How much CR does that add?
Well, it depends. If my monster was a huge dragon and it already had a bite attack that's way better than the greatsword, then the greatsword never gets used and it adds no CR at all.
On the other hand, if my monster was a ratling and its previous best attack was a dagger for 1d4+1, then the greatsword is actually a big deal.
So the answer I'd propose is: if you add an attack which the monster is going to use as its primary attack, you don't "add" to the Offensive Challenge for that. What you do is you figure out the Offensive Challenge for that attack, and you have just set the monster's Offensive Challenge to that number.
So: what is the Offensive Challenge of a petrification attack? What other monsters can we find that have petrification attacks, and what is their Offensive Challenge?
The basilisk has a similar power, except it's attached to a Constitution saving throw, DC 12. This power is the basilisk's most dangerous attack, so the Offensive Challenge of the attack should be about the same as the CR of the basilisk. The basilisk is CR 3, which, uh, seems kind of low actually for a save-or-die attack. The CR for your power might be lower still, since it requires an action to use and only affects one target at a time.
The medusa also has this power, except in the medusa's case the power is DC 14 and the CR is 6. Maybe we could make sense of this by saying that adding +2 to the DC means +2 to the CR, and the medusa also has a snake hair attack which is pretty dangerous, so maybe that increases the CR by 1 since the medusa can do both at once.
So I think the answer to your question will depend on the DC of the ability.
If I could add a note, as well: in my experience, fighting monsters with save-or-die attacks is frequently not fun. Either the player character makes the save and the monster's attack does nothing, or the player character fails the save and now that player is out of the game. You might consider replacing the save-or-die with something that does (for example) ability damage.