Retrieve the Sealed Azure Bag from the Timbermaw Shaman in Azshara. Then take the bag to Archmage Xylem, also found in Azshara.
First things first. What I've lost is a small blue bag, the contents of which are a private matter.
Don't bother going after the thieves, because they are already dead. What we do know is the fools couldn't get the bag open. so they sold it to some timbermaw shaman in Azshara as a holy relic. Clever.
I need you to travel to Azshara and retrieve it from the shaman through subtlety... or force. Better luck with the first method, I'd wager. Once acquired, take it to Archmage Xylem.