I started the fight against Pontifex Aurelus with my Paladin, and after ~10 seconds, a huge grey/black AOE appeared around my character and then ennemies as well as my character became (almost) immune to any damage.
I was able to stand still and took litteraly no damage, while able to only deal damage 1 by 1.
Here is the video : Last Epoch : Pontifex Aurelus bug - YouTube
I logged out to the character selection and come back and it was fine.
Have a good day.