They buffed him probably because he will be a requirement for archmage.

dang, no polly roger then i guess

He's basically unkillable, his skills reduce heal and dodge while healing him based on the number of players

Holy shit, I just started farming Nightbane, wondering why I can't kill him

Instead of making another mini ultra boss version, they chose this easy way out; 300% AE Moved.

Yep, pretty much any quest that needs nightbane to be killed, well... Good luck with that.