amenazar, verb:

to threaten

Amenazar is the basic word for to threaten somebody. Remember to use the preposition a before the person being threatened:

Se dedican a amenazar a los vecinos.
They spend their time threatening the neighbors.

Notice the construction amenazar de muerte a alguien to threaten to kill somebody.

acusado de amenazar de muerte a su ex mujer
mujer accused of threatening to kill his ex wife

Sometimes a verb in Spanish uses a preposition you might not expect. If you threaten to do something, the phrase is amenazar con hacer algo:

Amenazaba con dimitir.
She was threatening to resign.