General terms:
* Demon
* Evil spirit
* Fiend
* Imp
* Hellhound
* Devilish
* Diabolical
* Infernal
* Malicious
* Wicked
* Sinful
* Cunning
* Devious
* Treacherous
* Tempting
Physical descriptions:
* Horns
* Claws
* Hooves
* Tail
* Wings
* Red eyes
* Ugly
* Grotesque
Personality traits:
* Cruel
* Sadistic
* Ruthless
* Vengeful
* Envious
* Greedy
* Lustful
* Prideful
Metaphorical/Figurative:
* A personification of evil
* A symbol of temptation
* An embodiment of darkness
* A force of chaos
* The antithesis of good
Specific terms:
* Lucifer (The fallen angel, often considered the Devil)
* Beelzebub (A demon often associated with Satan)
* Asmodeus (A demon often associated with lust and temptation)
* Belial (A demon often associated with wickedness)
The best words to describe a devil will depend on the specific context and the image you are trying to create.