All we require you to do, is stay in the guild the day before cataclysm releases (the 6th) till cataclysm "unlocks" at 12:00pm and the achievement is granted. Multiple legendary items would be great, however only 1 is required.
Require you to provide proof of legendary/s (Via armory). If for some reason the achievement is not granted upon release, you'll be paid for your time/effort anyway.
Leave a message here, or send an in game mail or whisper to "Blankalt" for more information.