* An evil spirit or demon: 悪霊 (akuryō) or 魔物 (mamono) are good choices. 鬼 (oni) is also possible, but implies a more specific type of Japanese demon.
* An addict or someone with a strong craving: 中毒者 (chūdoku-sha) (addict), 耽溺者 (tan'nyaku-sha) (someone indulging excessively), or 熱狂的なファン (nekkyō-tekina fan) (fanatical fan) might be suitable depending on the context (e.g., a "chocoholic fiend" would be better served by 熱狂的なチョコレート好き).
* A wicked or cruel person: 悪党 (akutō), 悪人 (akunin), or 凶悪犯 (kyōakuhan) (violent criminal) could work, but these imply more serious villainy.
To choose the best translation, you need to provide the context in which you're using the word "fiend."