Natterjack implings are an extremely weak subtype of Imps, described as "slightly less dangerous than a piece of moldy bread" and unadventurous creatures "that affects the semblance and habits of a dull sort of toad". Arthur Underwood wanted to teach Nathaniel to summon, starting with one of these, not knowing he had actually already summoned Bartimaeus, a considerably more powerful spirit to say the least.