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The shadow. Not really what you are looking for but dangerous to all levels
Low CR, stat draining abilities
The look on your players face when their Dex fighters strength goes to zero and they find out they are dead
We had a gamemaster ban humans, the result everybody chose different races. Just let everyone know before characters are thought about what are the options and as long as everyone is reasonable it will work. D&D is very resilient.
The one I saw played like a fighter with a d6 hit die. They did o.k. until they gained the ire of the foe then they spent the rest of the battle making death saves. (or trying to make death saves) AoE's are their doom