Once, many years before the beings known as "The Devs" came to Draenor, the Temple of Karabor was the Mecca of the Dranei's spiritual life. Then, of course, the peaceful cloven-hoofed priests were slaughtered by orcs hopped up on demon juice, and the Shadow Council's warlocks rechristened Karabor with a sinister, conveniently simple new name: The Black Temple!
And today that invasion is mirrored by a 250+MB download that shall invade your hard drive and update your version of World of Warcraft to 2.1.0, while opening the gates to the Black Temple in Outland, where the demon general Magtheridon laired until no less a bad guy than Illidan the Betrayer ousted him.
Long story short: the Black Temple is open and ready to kill you.
Also along for the ride with this mega-update are over 100 new rare and epic items and tier-6 armor sets, nestled unquietly within the massive, non-linear 25-man raid dungeon. Also new: druid epic flight form; a Netherwing faction quest extension and the availability of the Nether Drake as a 280% faster epic flying mount.