The eighteenth trump is the strangest picture in the deck and it has been strange from the beginning. The French pattern shows two dogs or wolves baying at a moon that has a face, drops falling from the sky, two towers behind, and a crayfish or lobster in a pool in the foreground. Contrary to a common assumption, none of that is a modern invention. The crayfish and the dogs are in the older cards; Pamela Colman Smith rearranged the composition in 1909 but did not create it.
What nobody can supply is a source. There is no medieval commonplace that assembles these particular elements, no obvious allegory, and no text that explains the crayfish. Proposals exist, a lunar eclipse diagram, an astrological illustration, a scene of the dog-days, the moon drawing water from the earth, and none is established. This is one of the few tarot images for which the honest answer is that its origin is not known.
The Golden Dawn assigned it the letter Qoph, the sign Pisces, and the twenty-ninth path from Netzach to Malkuth, which is the lowest and most material of the paths connected to the personal sphere. Qoph is usually read as the back of the head, and the order connected the card with the parts of consciousness that are not available to inspection. That is a good fit for the picture and, like every such fit, it was arrived at long after the picture existed.
Waite's own commentary on the card is unusually murky even by his standards, which may be the most honest response available to a picture nobody has satisfactorily explained in six hundred years.