adversary, enemy

Definition:

An "adversary" is a person or group who is opposed to someone or something. The term "enemy" has a similar meaning.

Your adversary can be a person who tries to oppose you or harm you.
When two nations fight, each can be called an “adversary” of the other.
In the Bible, the devil is referred to as an "adversary" and an "enemy."
Adversary may be translated as "opponent" or "enemy," but it suggests a tronger form of opposition.