Hey @Martentos ,
@mitamata, @Shimrra and @alv-thanos beat me to it 
Its’s as they said Essence of Evil applies curse which nullifies the Impervious trait, allowing Zuul’Goth to get Stunned. This then allows other negative status effects to be applied.
Looking at the Impervious and Essence of Evil’s descriptions, I can see how the wording can be confusing and providing a clear explanation on how it’s intended to work. I’ll pass this feedback on to the Development team to make some quality of life improvements to make them more clear.
Thanks for bringing this feedback up, it’s much appreciated 
OminousGMan - Support Human 