Hide a Brightly Colored Egg in Stormwind City.
The Noblegarden Egg is a user-dropped version of the Brightly Colored Egg. It can be purchased for five Noblegarden Chocolates during the holiday Noblegarden. It functions just like a Brightly Colored Egg, in that inside will be a chocolate and a chance at loot, and anyone can loot it (including you). Just purchase your egg and head on over to Silvermoon City to drop it to complete this achievement.
The Horde equivalent achievement is Noble Garden.