The Courier Drake allows you to send half of the Crystals on your watch to the Black Market on your phone to be spent. It can be used once per generation. It can be obtained with Diamonds in the Magic Shop. Once obtained, your Heroine will always have this item.
This should be the last item in the Magic Shop that you spend items on. It provides little value for speed runs and minimal value for pushing runs. But if you have the diamonds to spare, why not? Otherwise, save your diamonds for something more useful.