a cruel or evil person
fiend - monster
A ‘fiend’ or something ‘fiendish’ is typically characterized by wickedness, maliciousness, or intense difficulty. However, the word can also be used in an informal way to refer to somebody who is extremely interested in a particular thing and enjoys having or doing it to an excessive degree. Depending on the thing in question, this sense of the word can have highly negative connotations.
The prominent business magnate is one of the biggest employers in the country, yet the media relentlessly portray him as a heartless fiend.
Tom is such a sugar fiend that he finds it very difficult to stick to a diet. He just craves sweets all the time!
A number of senior figures within the company hatched a fiendish plan to oust the CEO from power. It was driven simply by greed and self-interest.
I’m not looking forward to sitting my entrance exams later this month. They are known to be fiendishly difficult.