D&D 5E Help me stat the Offensive CR of a monster with at-will 9th level Magic Missile

[Stalker0]

So I have a custom monster I've been brewing for a high level game.

The offensive attack of this monster is a ranged claw attack (basically he can swipe midair, and slashes will appear on you). This is just a re-skinned 9th level magic missile.

The party does not have any particular resistances to force damage. So the creature will do an average of 38.5 force damage, nearly guaranteed, every round.

That puts it at a CR 6 monster by damage. However, of course you have to factor in attack bonus....and this attack is technically infinite. Auto attack is very strong, but at the end of the day its not that much damage....and you also have lots of spells that do half damage. High level adventures easily have fire resistances on big blasting spells, but force resistance is rare.

So where would you stat the offensive CR of this critter?

 

[UngeheuerLich]

CR x means it could kill a char of level x-1 outright with a single attack. Lets take 1d6+2 hp per level as the floor. So 6 average hp per level. It does slightly over 36 damage. So the monster should at least be CR 7 just for offensive CR.
If I didn't do it wrong standard deviation is about 7.5. So its not too uncommon that the monster does 38.5+7.5=46 damage.
I would probably make it offensive CR 8 to have a little buffer.
And two attacks do an average of 77 damage and usually at least 77-10.5=66.5 damage. An average fighter with con 14 would have 8hp per level. That is 64. His starting hp would just bring him to 68hp and second wind might allow him to survive the second attack.

The only character who has a good chance of survival would be a bear totem barbarian and someone with the shield spell if you allow it to work against the claws.
High mobility though allows for hit and run tactics and barbarian rages at that level and even spell slots are not endless.

A different train of thought might be that a +4 or 5 bonus to hit will bring the standard attack bonus of a CR 6 creature which should be +7 or so to +11. Which is nearly auto hit on the lowest armored AC 2+11=13 characters. (A 1 would fail anyway). +2 to hit results in CR+1.

 

[FrogReaver]

So I have a custom monster I've been brewing for a high level game.

The offensive attack of this monster is a ranged claw attack (basically he can swipe midair, and slashes will appear on you). This is just a re-skinned 9th level magic missile.

The party does not have any particular resistances to force damage. So the creature will do an average of 38.5 force damage, nearly guaranteed, every round.

That puts it at a CR 6 monster by damage. However, of course you have to factor in attack bonus....and this attack is technically infinite. Auto attack is very strong, but at the end of the day its not that much damage....and you also have lots of spells that do half damage. High level adventures easily have fire resistances on big blasting spells, but force resistance is rare.

So where would you stat the offensive CR of this critter?

So basically the creature can kill a level 4 bard in 1 hit and is guaranteed to hit. He can kill a level 5 wizard in 1 hit if that wizard doesn't have shield (assuming shield works on the claw attack like it does magic missle). He can even kill a level 4 fighter or paladin in a single hit. Unless we are talking a bear totem barbarian pretty much every character up to about level 8 can be downed in 2 turns to that creature. If the party is unlucky and misses their first attacks/spells that creature may cause a party wipe on a level 4-5 party pretty easily. Taking down 1 character a turn will be rough.

I'd say consider it's damage as somewhere between 50% more and double what it is listed at due to the auto hit nature of the ability. I'd say that's the appropriate offensive CR for the creature.

 

[UngeheuerLich]

So basically the creature can kill a level 4 bard in 1 hit and is guaranteed to hit. He can kill a level 5 wizard in 1 hit if that wizard doesn't have shield (assuming shield works on the claw attack like it does magic missle). He can even kill a level 4 fighter or paladin in a single hit. Unless we are talking a bear totem barbarian pretty much every character up to about level 8 can be downed in 2 turns to that creature. If the party is unlucky and misses their first attacks/spells that creature may cause a party wipe on a level 4-5 party pretty easily. Taking down 1 character a turn will be rough.

I'd say consider it's damage as somewhere between 50% more and double what it is listed at due to the auto hit nature of the ability. I'd say that's the appropriate offensive CR for the creature.

So I looked it up: If you assume +50% damage, you are at the bottom of CR 9. CR 6 would only be an attack bonus of 6. So adding 6 would be an attack bonus of 12 nearly auto hitting AC 14. Might be a better estimation for an auto hit attack and so I would probably support CR 9 with that approach.
Also CR calculations for spells assume 3 rounds of spells with failed saving throws which you do in about 2 of 3 times (DC 15). Failing all would be +50% damage and you need a DC of 21 for a spell to be auto hit. If a spell would deal half damage on a miss it is only an increase of 20% though. 6/6 instead of 5/6 of the maximum damage. So maybe CR 8 would be more appropriate.

 

[Xeviat]

Interesting question! I like the CR 8 assumption by the calculations here.


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