Nurgloth the Eternal

Attacks
First half of the fight he alternates between being out of range while you fight off hordes, and coming down to call forth rings of fire. Avoid the fire by standing outside of the glowing orange rings.

Second half of the fight he flies after players while swinging his scythe.

Strategy
Damage it to lower its health.

Keep an eye on the ground. Nurgloth's magic will create yellow rings on the ground before they explode. While the area directly around Nurgloth is not glowing, it is safe to move in and attack with melee.