Historically, most tombs known to have grave goods in them were robbed soon after the dead person was laid to rest. So why has the tomb that your group of adventurers broken into not already suffered this fate?
Every now and again, the region encounters a dense blue fog. Every now and again, things disappear into the fog never to be seen again and things appear out of it from other places or world. It seems the fog is the material form of some minor diety charged with moving things around the universe to balance cosmic forces.