The Lord Ahune encounter is very easily one of the most enjoyable 5-man encounters we've seen in a very long time. The Burning Crusade added some elements we had never seen in the pre-expansion days, and I feel that this boss takes it much further than that.
Ahune is found within the Slave Pens, and while he's fairly close to the beginning of the instance, you'll need to do a little bit of clearing before you fight him. Since Slave Pens is one of the easier dungeons in The Burning Crusade (both Normal and Heroic versions), you shouldn't have much trouble with this step. Once you've reached the area where you summon Lord Ahune (and you will know when you get there), buff up, drink up, and make sure your party is ready. While not the most difficult encounter in the world, this alternating two phase boss fight can drag on a really long time if you're not ready.