He is depicted as a phoenix, which sings sweet notes with the voice of a child, but the conjurer must warn his companions (for he has not to be alone) not to hear them and ask him to put in human shape, which the demon supposedly does after a certain amount of time.
Johann Wier's Pseudomonarchia Daemonum describes this spirit as follows:
"Phoenix is a great marquesse, appearing like the bird Phoenix, having a child's voice: but before he standeth still before the conjuror, he singeth manie sweet notes. Then the exorcist with his companions must beware he give no eare to the melodie, but must by and by bid him put on humane shape; then will he speake marvellouslie of all woonderfull sciences. He is an excellent poet, and obedient, he hopeth to returne to the seventh throne after a thousand two hundredth yeares, and governeth twentie legions."
Other spellings: Pheynix, Phoenix, Phoeniex.