First off, I'd like to warn everyone that this post contains fairly detailed spoilers for the upcoming holiday, the Midsummer Fire Festival. If you'd like the details of the new boss to remain a surprise, this post probably isn't for you! But I know quite well that most WoW players can't stand not knowing what's what, so I'm sure many of you will continue.
As mentioned in Daniel's post a few days ago, this Festival will bring us another Headless Horseman-esque boss, complete with phat lewts. You can check it all out on the PTR right now. Found within the Slave Pens, this boss, named Lord Ahune, is another Ragnaros look-alike. As you can probably guess from his appearance alone, Ahune is another lieutenant of the Elemental Lords, like Prince Thunderaaan. Ahune is serving one of the Lords in particular: Neptulon. Is this another bit of the story of Skar'this? Time will tell!
The Lord Ahune encounter is an alternating two phase battle, again similar to the Headless Horseman. Without giving away too many details, be prepared to fight a number of adds in one of the two phases and have some burst damage queued up for the other. Most any group worth their salt should be able to put away Ahune relatively easily.
While the battle is fun, the loot is funner. More fun? Funnerer.