The prize for this week's most-talked-about Sunwell boss goes to... an imprisoned naaru. Formerly resident in Silvermoon City, in patch 2.4 M'uru was captured by Kael'thas and taken to the Sunwell Plateau, where he now holds the place of fifth boss in the instance. After defeating the Eredar Twins (which gives you a handy teleport to their room) and clearing a few trash packs, you'll find yourself at the doorway of M'uru's circular room.
Initially a fairly unimpressive looking boss -- discounting the 'wow, it's a naaru' factor -- as the encounter gets into full swing you begin to marvel at its clockwork complexity. Killing M'uru isn't the end of the fight, either; instead of dying, he transforms into a void god called Entropius, who you have to burn down as quickly as possible. It's a very intense encounter, even post-nerf.