Where did you see the imbalance here?
It's so imbalanced that physically powerful level 6 creatures can take down an entire creature, even though their attack is many times stronger, and it doesn't matter at all whether one creature or a million uses this ability? So far, I haven't seen any creatures other than the ones mentioned above that have this ability. And what you're suggesting would make it completely useless. By the way, it doesn't work on mechanical or machine creatures.
Besides, there's no way to change the code of how abilities work (we tried, but it didn't work). We can only change some numerical parameters.
But what if it's 1v1 in the literal sense? Then Undead can take down even a Cyclops or a King of the Universe, and that's not right, and that's what I'm getting at.
Added 3 minutes later
In general, according to Folklore, plague is death that reaps the weak – like a virus, while the strong remain alive. So, you could make it so that it decimates the weak by +10, but the ability doesn't work on strong creatures, then a certain balance would be maintained, and the ability wouldn't be a throwaway. And there would be some realism.
