In demonology, Phenex is a Great Marquis of Hell and has twenty legions of demons under his command. He teaches all wonderful sciences, is an excellent poet, and is very obedient to the conjuror. Phenex hopes to return to Heaven after 1,200 years, but he is deceived in this hope.

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 on a human shape, which the demon supposedly does after a certain amount of time.

The Lesser Key of Solomon 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 many sweet notes. Then the exorcist with his companions must beware he give no ear to the melody, but must by and by bid him put on humane shape; then will he speak marvelously of all wonderful sciences. He is an excellent poet, and obedient, he hopeth to return to the seventh throne after a thousand two hundredth years, and governeth twenty legions."