The necromancer's name is Kotiz the Death Bringer. He's an 86^^^ heroic. He is the one in front of Venril Sathir's Lair zone in. He's only a 2-3 hour timer and his kill update is shared with at least three other classes. The ring is a corpse drop and if ANYONE touches the body but you, it will despawn!!!
Apparently Praetorian Captain Nagol at -15, -129, 463 (a Praetorian officer is the PH) in the previous room can drop this update, as well as Nihilist Yeegarn at 72, -131, 164 (a cabilis resident is the PH) and Nihilist Zeegarn0, -136, 186 (a cabilis resident is the PH) in either of the two entry rooms to the Ruins of Sebilis (basement)