Hello everyone,
I have a small speculation, is it possible that Bownsamdi boss is actually Ner'Zhul ? Maybe Bolvar Ended up by getting corrupted after all this time Wearing The Helm Of Domination ?.
Furthermore, upon the Lich King's death he even spoke of "Seeing only darkness before him" while Sylvanas Windrunner said the same in Silverpine Forest after being risen from the dead by the Val'kyr. This may hint that because undead, death knights, and necromancers are connected to the Realm of Shadows when they die (again) they actually become apart of the Realm of Shadows and are forced to wanders in its dark mists for the rest of eternity.