Stone citadel boss fight , Permanent mana drain: is this a bug or intended?

So my first time fighting the stone citadel boss and I watched a few videos but I never saw him doing this: During the 2nd phase he had a huge mana drain aura around him the ENTIRE 2nd phase. If I tried going through the blue ring and get close, it still would drain all mana instantly and continue to do so. As melee I could really do nothing. Was this intended? I started to leave and reset but got killed while trying. Just wondering if anyone has encountered this perma-mana drain during the fight?

You have "Deadly Evolution" on your atlas passive tree, it adds mods to bosses.

Also only the thick border of the circle drains mana, you can stand inside it or outside it and it wont drain mana.

Edit: If it was doryani maybe his big size prevents you from going inside :/

Thank you for the reply. I think you are correct about the deadly evolution and I did have it on my atlas tree. He did have a thick blue circle you mentioned. I tried going inside next to him and hitting but the mana drain kept moving with him which made it nearly impossible to avoid it and it dropped me from 900 mana to 0 in 1 second. I am taking deadly evolution off permanently after this.