Its not a beta final event or a quest coming up to lich king. It is a quest that starts in Dragonblight after you complete a bunch of quest chains at different hubs. After you do this you are to report to saurfang junior at the wrath gate. He sends you to alexstraza to do some quests then you go back to him. Then the game uses the mechanic as the DK starting zone where its almost like an instance zone and things change. You then get sent back to org/stormwind and learn about what is going on in UC. You then get a quest called The Battle for Undercity.
It is around a level 75 questchain that is truly the most epic chain you will do in WoW. I think I gave a decent description without giving away any of the story behind the quest because it is pretty awesome. It is also probably the only time WoW you will be close to god mode.