The Fallible Fiend (Signet Q5370)

Feb. 1973 Signet mass market paperback, L. Sprague DeCamp (Conan the Warrior). On the demonic Twelfth Plane the demon Zdim is drafted for a year's indentured servitude on the human Prime Plane, the demon society having an agreement to provide service to human sorcerers in return for supplies of iron, a raw material it desperately needs. Zdim is duly summoned to the Prime Plane by the sorcerer Dr. Maldivius of Novaria. There he strives to do his duty, but his demonic literal-mindedness hampers him. Assigned to protect the Sibylline Sapphire from any trespassers, he promptly eats Maldivius' apprentice Grax when the latter intrudes. - Wikiepdia