Usage patterns
魔物が現れるa demon/monster appears
A demon appeared from deep within the forest.
魔物を退治するto defeat/exterminate a demon
The hero defeated the demon that was attacking the village.
魔物に取り憑かれるto be possessed by a demon
He suddenly went berserk as if possessed by a demon.
魔物の仕業the work of a demon (blamed for something ominous)
This eerie incident is surely the work of a demon.