I have left this alone for a while to give other players a chance to speak.
Here are a few ideas:
Mist form: negates smashing hammer by X %
Uncanny anticipation : negates Flinch by X%
Sliding Scales: Negates Keen edge by X %
Witch's eye/God's blessing: Negates Dark Curse by X %
Aura of Aggressive Disharmony: Negates co-ordinated attack by X %
Aura of Defensive disharmony: Negates co-ordinated defence by X %
Aura of disharmony: negates both co-at and co-def by X %
Love you to Death: negates Nightmare Visage by X%
And so on and so forth for all stat boosting buffs/enemy stat reducing buffs. I would exempt base stat boosting buffs from this. (Berserk, Evade, etc.) Similarly I would not pursue things like e.g. Disruptive Blow to reduce % effectiveness of Force Shield etc., because those are secondary effects, rather than primary force multipliers, and so should be ignored for now. (IMO)
The idea would be to use these on SELECT monsters at game critical junctures. Not to spam them like the Souless enhancement that basically negates wither all the time after wither was made too powerful.
I would introduce them as part of a story cycle involving either Morbidstein (my fave!) doing new experiments to slow down our heroes, or Xind, or Lishka, any classic enemy to wrap a story around. The Shadowlord could respawn the Dreadlord with new lesser minions with some of these enhancements, designed to see how the heroes would deal with these greater challenges. A sequence of levels with harder foes and higher enhancements could easily follow.
Or, as I suggested years ago, it could be part of a shadow lord content event, as we draw closer to him, or an area precious to him, the monsters grow stronger and have increasingly strong %'s of some of these enhancements (typically designed/related to their stat distribution to make them as challenging as possible). The characters pass through veils of shadow, and so must defeat monsters that can negate (partially to fully) the hero's spell and potion gained abilities.
But it should never be used like Souless to 99% nerf Wither all the time. That is boring. Use stats plus enhancement variation to make it tricky for players to anticipate what to do. Make the enhancement variable on a range to screw with expectations, sure, but don't go straight to nerf except on very specific levels you want to make EOC suffer through, and it should be obvious you want them to struggle through those levels by story element guidance. In my opinion.
For example:
Random level 5827 monster that nerfs most bonus buffs is worthless.
But a 5827 monster that nerfs some buffs a bit because it guards a gateway to 5828 filled with only three shadow elites (high stat nightmares) that each have separate total nerfs to crucial hunting buffs that takes you to 5829 and a monster with a collection of nerf enhancements that guards a quest locked door and must be killed 2000 times to advance to 5830 and a boss monster SE... sweet. Make people 2 hit. Make folk consider suicide set ups to max XP gain.
The best version of that would be for the above sequence to be stretched over 50 -100 (or more!!) levels with different combinations of enhancement potency, quest goals, story elements, and boss/ level advancement reveal.
The new enhancements would then ONLY be used for special content, or themed sections. Hallowe'en and Christmas variants could be created easily.
Please do not create new enhancements and then just apply them to all new monsters, nerfing huge swathes of buffs all the time. That would just suck.
That's the idea there. It is rough, and I'm sure unintended consequences abound everywhere. What do folk think?