Defeat Doomwalker in Tanaris.
Did you see that giant mechanical monstrosity just walking around in the dunes outside? He comes from a place out of time, and not even here on Azeroth! The timeways sure are screwy around this time of year. <Chromie takes out a small, rapidly whirring stopwatch and tinkers with it. She seems to have momentarily forgotten about you.> Oh, you're still here! That's great. Would you mind taking out that thing? Otherwise, people might have trouble getting to the party!