This is one of the movable adventures for Egypt. It can be in the basement of a house, under the market or in the tomb by the basecamp. You'll need to look for a map tag to tell you where to go. Then just explore the tomb until you find the chest with rays of colored light coming out of it. It will be in there.
Comments
I've had it given to me twice. In the same day. From two different sims, but the same exact location.