According to Johan Weyer, Bathym is a duke of the infernal regions, also known as Marthin. He has the appearance of a robust man, but his body ends in a serpent's tail. He rides a steed of livid color and is able to transport men from one place to another with wondrous speed. He is well versed in the virtues of herbs and precious stones. Thirty legions obey his behests.
Weyer, Johannes. Witches, Devils and Doctors in the Renaissance: Johann Weyer, De Praestigiis. Edited by George Mora. Binghamton, N.Y.: Medieval & Renaissance Texts & Studies, 1991.