By the time you hit level 64 you should have explored all of the Hellfire Peninsula, Zangarmarsh and parts of Terokkar Forest. Once you start exploring the forest, you will come across the vast Bone Wastes in the south of the zone and spot the ruins that are Auchindoun in their center. Auchindoun is a huge structure that holds 4 different instance in it that are meant for levels 64 through 70. The first one you should explore is the Mana Tombs.

The Mana Tombs themselves are a wing for players level 64-66 that mainly house energy mummy type creatures called ethereals. Outside the Mana Tombs you will find more of these creatures, but they are an opposing faction called the Consortium. The Consortium is an organization of ethereal traders, smugglers or thieves (depending who you ask), that would like you to enter the tombs and gain them access to it, since the other ethereals have set up a blocking field to them of some sort.

You can read the whole guide here: TenTonHammer's Guide to the Mana Tombs.