Welcome Vesalius, the first new Rift Boss in a long time! While it seemed he was entirely unique at first, it later turned out, as most Rift Bosses, that he was based off an Adventure Mode boss (Vidian from the Sanctum of Blood, a new area). Both bosses are giant Enslaved Nightmares, new models coming with the Necromancer pack and showing up now on the beta, so we can hope there will be even more new additions once it goes live.
Streamer Zhanji first ran into the boss and his Necromancer's first encounter with Vesalius didn't go that great, considering the Frozen Pulse, Wormhole and Glowy Balls. So, let's take a look: