A person who is considered a lifeline threat or adversary.

A rivalry characterized by animosity that could lead to serious conflict.

दुश्मनी

English Usage: Their rivalry turned them into mortal enemies, constantly competing against each other.

Hindi Usage: उनकी प्रतिद्वंद्विता ने उन्हें दुश्मनों में बदल दिया, जो लगातार एक-दूसरे के खिलाफ प्रतिस्पर्धा कर रहे थे।