The Lost Crypts are some crypts which Grayson wants to find because they apparently contain great riches. One of them so far has been found, near Uldun and it's Desert Temple. It was a smaller crypt, not one of the Great Lost Crypts, and contained many zombies, plus a map of a nearby location. There was also a random lever inside, which did nothing. Grayson commented upon discovery that it probably was not a Great Lost Crypt, and instead was just a minor crypt.