Determining New Zombie CR's for 5e

[PhoenixPhyre]

An aoe, friendly-fire-free (if they're running with zombies), no immunity stun that deals damage, even with a low DC, is a hefty effect. And one that CR (in particular) is not well equipped to model/correct for.

Anyone with +2 CON and no save proficiency has a 45% chance of losing their next turn and getting smacked by all the other zombies. In a 3 round fight, that means that there's the following breakdown:

Rounds stunned | Chance
0 | 16.6%
1 | 40.1%
2 | 33.4%
3 | 9.1%

So there's a 77% chance you will spend either 1/3 or 2/3 of the fight out of combat. That's brutal. And not so fun. Hard control (complete action denial) is rarely very fun at all for the players. You're basically saying "don't play a melee character" here.

Personally, I'd downgrade it to something like a piece of what slow does (reduced movement, action OR bonus action) or add in a "if you succeed, you're immune for X time" clause.