noun
BAD : They used to threat each other with atomic bombs.
GOOD : They used to threaten each other with atomic bombs.
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Threat is a noun: 'The letter was full of threats and accusations.' 'The President said he would stand firm and not give in to threats from terrorists.'
The verb is threaten : 'Whenever they have an argument, she threatens to leave him.' 'He is threatening the magazine with legal action unless they publish a full apology.'