"This new boss can be fought with any Combat Style across three difficulties - Story, Normal and Hard mode - alone or with a group - with new Ranged and Necromancy drops to pursue"
With story, normal and hard modes, this is the format of an Elite Dungeon. There may be multiple bosses and drop multiple ranged and Necromancy items.
I'll guess they drop materials like in ED1 to ED3. We craft them into different items. We don't craft Necromancy items though, so we probably do rituals on the Necromancy drop.
Yes, most likely like the melee glass cannon gear. All 3 modes from story to normal to hard mode drop the gear, just higher drop rates at normal and hm. I'm guessing the vorkath and zemouregal fight will also drop a few codices and a new bis necro item.
Following the early ED drop format, trash mobs can drop important materials for the new gear, as well as old content too, so perhaps bosses drop more new gear materials than trash. New ability codice among other non-gear items are boss only drop probably though.