Pretty sure it will only be as part of a dungeon, and seeing as the dungeon is not available yet I am going to go out on a limb and say that no, noone has seen the Naaru yet.
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Speaking of Denathrius. That sword he has looks eerily similar to Frostmourne, I wonder whether it will be a running thing where the various leaders of the Shadowlands has a named weapon, possibly even all having swords.