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?