demon
n. countable C1 Advanced Oxford US //ˈdimən// UK //dˈiːmən// de·mon
n. an evil spirit or a very cruel person. In stories, these creatures often cause trouble or try to hurt people.
n. an evil spirit or supernatural being, often depicted as a source of malevolence or temptation. In a figurative sense, it refers to a person perceived as exceptionally wicked or a persistent source of personal struggle.
The hero in the story fights a powerful demon.
The ancient legends describe a demon that guards the entrance to the cave and tricks travelers.
While the novel features literal monsters, the protagonist's true struggle is with the internal demons of guilt and regret that haunt his every decision.
Usage
Often used metaphorically to describe personal struggles or addictions ('fighting his demons').