Back in December 2021 illusionist grudge marked bosses were changed so they would not give illusions grudge marked traits but if they have cursed aura they do.
Chaos Wastes - Illusionist illusions no longer inherit other Grudge Mark properties, but they are still shown on the HUD (under the health bar) as if they do
How the curse works:
Gain 1 stack of curse every 2 sec if you are inside of Bosses aura.
Each stack reduces your max HP by 5%.
Can have up to 20 stacks of curse.
Curse expires after 5 sec not affected by the aura.
Warrior Priest is immune to this effect.
I got almost a full red bar of health from 4 minotaurs in 8 seconds and even if you kill the illusions its not like the red bar is going to disappear because you need to be outside of the cursed aura for 5 seconds before that happens.