When you finish all four wings, you get access to the final boss of the DLC, Ionos (the central column will rise, opening a portal).
The boss is quite difficult even on STORY difficulty. His stomp attack especially is quite devastating.
To defeat him easily, I suggest entering the arena with a full HAVOC meter and a few HAVOC shards ready. Beat the hell out of him, then recharge havoc with a readied shard (careful not to get one-shot with one of his stomp attacks) and rinse and repeat.
It helps if you have the Panoply of Champions armor, since it more than doubles your havoc time! (get it from the Crucible DLC)
(this tactic works with most bosses in Darksiders 3)