Determining New Zombie CR's for 5e

[Tevin]

Sound burst isn't a spell in 5e.

I think you're underestimating the durability of zombies--that undead fortitude means that they've got a decent chance to last longer than their other stats would suggest. Basically, unless this one gets sniped priority, it's likely to get off at least 2 shrieks. Which means there's a 70% chance that at least one person's going to spend 1 or 2 rounds stunned, which drastically increases the encounter difficulty (without really changing the monster's CR, due to how it's calculated).

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By the books, it does an additional (averaged) 0.833 DPR [(2d6[0] - 1d6 - 1)/3] compared to a regular zombie, with the rest being equal. I'm away from my data right now, but I doubt that that would push it over any boundaries. So not counting the stun effect, no change.

Throw in an ad-hoc +1-step CR for the stun effect (effectively being the same as granting advantage to all allies on half the party[1] plus a 50% damage reduction for one round), and you've got a CR 1/2 creature. But then I'd have to play-test and see how it really felt. Might be worth a +2.

But as I said, it's not something you can really apply the standard CR calculations to and hope to get a decent result that really tells you much about how it will handle in a group. Because CR doesn't take into account that kind of group synergy. Alone, the thing gets smashed into oblivion by a level 1 party, no issues. With 3-4 other zombies and you could very well lose a person in a level 2 or 3 party. Get lucky on recharges or on placement (ie if it doesn't make itself noticeable until it's right near a cluster of PCs) or on saves and you could have a TPK.

[0] assuming it goes off once and hits two people, who both fail their save.
[1] by the assumptions of the book, 2 people get hit and fail. That means there's at least one round where half the party is out of commission and can be smashed by the rest and can't deal damage.

Sound Burst is a 3rd party spell from Frog God Games.

I understand the potential survivability of zombies, I am running a zombie apocalypse game after all, and being 4th level now we have encountered quite a few zombies. This one would obviously get snipe priority since it would be the only special zombie amongst a group of regular zombies and especially so after the group has encountered it before. That doesn't seem like a design flaw but a good party tactic. In a 3 round fight there's really not a good chance of the ability recharging. It'd be 1/9 or 11.11111% of going off twice. Then when it does go off the typical adventurer succeeds 45-50% of the time anyways. So in a given encounter, chances are, you'd have a 55% chance of costing the party 1 PC worth of actions (provided it hits half the party) and deals roughly 5 damage. After that there's an 11.1111% chance that it costs the party 1 more round of PC actions and deals another 5 damage, or it's effectively just another CR 1/4 zombie.

What are you arguing, that any AoE stun effect is too powerful on a monster?