As mentioned, if you change one special ability, you change them all.
The Drunkard is already somewhat overpowered.
And I wouldn't want to give a copy of a special ability.
I have a rule about high-level creatures: no heroes should be given.
Even if they don't have any special abilities.
To get a lot of Death Knights, it's better to focus on lower-level creatures.
For example, there are bears and lizards.
P.S. There's a hero among humans who's like an angel ;)
And what's wrong with that?)
So, if I persuade/somehow find/defeat 300 bears, will I get about 30 Death Knights? I wasn't even thinking about bears, then it would fit in his description: "upon destroying equine creatures and riders."